He was normally entirely ____________but in the embarrassing situation in which he found himself he felt compelled to ______________.
Forthright, prevaricate
Note that the structure of the sentence demands that the two blanks must be filled with antonyms ( 'but in the embarrassing situation' requires that the person does something contrary to his usual behavior )
Considerate means 'understanding'. So the person must behave in an 'inconsiderate' or 'harsh' way. Capitulate means to cease to resist an opponent or an unwelcome demand. So option A is incorrect.
Similarly forthright and concede in option B are not antonyms nor are Equable and concede in option D.
'To prevaricate' means 'to lie'. So a normally forthright person may feel compelled to lie in an embarrassing situation. Hence, the correct answer is option C.