Let's carefully analyze the seating arrangement based on the clues provided.

Problem Restatement

• Persons: o, p, q, r, s, t, u, v, w, x, y, z
• They are seated around a triangle-shaped chair arrangement.
• Each edge of the triangle has 3 people seated.
• Each corner has 1 person seated.
• Total seats = 3 edges × 3 people per edge + 3 corners = 9 + 3 = 12 seats (matches 12 persons).

Key Points and Clues

1. Edges and corners:
   • Each edge has 3 people.
   • Each corner has 1 person.
   • So, on each edge, the middle person is "in the middle" of the edge.
2. Positions of o, p, q:
   • o, p, q are seated in the middle of each edge.
3. Positions of x and w:
   • Only 5 people sit between x and w.
   • Neither x nor w sits at corners.
   • t is second to the left of w.
   • t is immediate left of r.
   • u is immediate neighbor of x.
   • Only 2 people sit between t and u.
4. Positions of s, z, v:
   • s sits at one of the corners.
   • s is third to the right of w.
   • s is immediate neighbor to both z and v.
   • v is immediate neighbor to p.
5. Positions of y and o:
   • Two people sit between y and o.
   • o doesn’t sit near w.

Step-by-Step Deduction

Step 1: Triangle Seating Setup

Label the corners as C1, C2, C3. Between corners, edges have 3 people each:

• Edge1: C1 - a - b - C2
• Edge2: C2 - c - d - C3
• Edge3: C3 - e - f - C1

Total seats: 3 corners + 3 edges × 3 seats = 12 seats.

Step 2: Place o, p, q in the middle of edges

Since o, p, q are in the middle of edges, they must be in positions a, d, or f (the middle seats on each edge). Assign:

• o at a (middle of edge C1-C2)
• p at d (middle of edge C2-C3)
• q at f (middle of edge C3-C1)

Step 3: v is immediate neighbor of p

p is at d (middle of edge C2-C3). Neighbors of p are c and e (the seats adjacent to d). So v is either c or e.

Step 4: s sits at one corner and is immediate neighbor to both z and v

• s is at a corner (C1, C2, or C3).
• s neighbors z and v.
• Since corners have 2 neighbors (one on each side), z and v must be the two neighbors of s.

Step 5: s is third to the right of w

• w is not at a corner.
• s is third to the right of w.

The seating is around the triangle, so "third to the right" means counting 3 seats clockwise from w to s.

Step 6: x and w are not at corners

• x and w are somewhere on edges but not corners.
• 5 people sit between x and w.

Step 7: t is second to left of w and immediate left of r

• t is second to the left of w.
• t is immediate left of r.
• Only 2 people sit between t and u.
• u is immediate neighbor of x.

Step 8: Two people sit between y and o, and o doesn’t sit near w

• y and o have two people between them.
• o is at a (middle of edge C1-C2).
• w is not near o.

Final Deduction: Who sits at the corners?

• s sits at one corner.
• The other two corners must be assigned to the remaining persons who fit the constraints.

Since s is at a corner and neighbors z and v, and v neighbors p, the corners are:

• s at one corner
• The other two corners must be assigned to persons who are neighbors to the edges and fit the constraints.

Answer to the Question:

Who among the following sit at the corner?

• s (confirmed corner)
• The other two corners are not explicitly named in the clues, but since x and w are not at corners, and o, p, q are in the middle of edges, the remaining corners are likely to be:
  • The corners adjacent to s, which are neighbors to z and v.

Thus, s is definitely at a corner.

Summary:

• s sits at a corner.
• The other corners are not explicitly named but are neighbors of s, likely z and possibly another person.
• From the clues, only s is explicitly confirmed to be at a corner.

If you provide the list of options, I can specify exactly who among them sits at the corner.