The correct option is B humble
The word 'lofty' refers to being of a noble or elevated nature.
Option A, puny refers to being small and weak.
Option B, humble refers to being of a low social, administrative or political rank.
Option C, insignificant refers to being a person without power or influence.
Option D, casual refers to being relaxed and unconcerned.
Thus, according to the meaning, the word 'humble' is the opposite of the word 'lofty'.