Explanation

(c) The walls will act as two mirrors inclined to each other at $90{^\circ}$ and so will form $\left( \frac{360}{90} - 1 \right) = 4 - 1,\ i.e.,\ 3$ images of the person. Now these images with person will act as objects for the ceiling mirror and so ceiling mirror will form 4 images further. Therefore total number of images formed $= 3 + 3 + 1 = 7$
Note : He can see, 6 images of himself