According to the search results, the shape of the f-orbital is complex and does not have a definite shape. The f-orbital has 15 protons to complete a fifth level of a tetrahedral structure, and it follows the same rules based on proton alignment as the p and d orbitals. The f-orbital has seven different shapes, which can be seen in a picture. The f-orbital is dispersed, and the value of l=3 for the f-subshell makes it complex in nature. Therefore, the answer is option D, "No definite shape".