The sum of the first 121 natural numbers is given by the formula:

S=n(n+1)2S=\frac{n(n+1)}{2}S=2n(n+1)​

where n=121n=121n=121. Calculating:

S=121×1222=147622=7381S=\frac{121\times 122}{2}=\frac{14762}{2}=7381S=2121×122​=214762​=7381

The unit digit of 7381 is 1. Therefore, the unit digit of the sum of the first 121 natural numbers is 1