Mortal means someone who has to die or cannot live forever, e.g., humans. So, its antonym is immortal which means never dying, living forever.
Luminous means glowing, shining, etc. So, its antonym is Dim as it means dark or dull.
A pit is a hole, cavity, etc. So, its antonym is peak which refers to the top of a mountain or hill.
'Rotund' means curved or round. So, out of the given options, only thin can be its antonym as it means slender, slim, etc.
'Common' means ordinary, widespread, etc. So, its antonym is rare as it means unusual, uncommon or infrequent.