Which words cannot be changed when paraphrasing this text? A. "monumental task" B."handsome building"C. "fifteen designs" D."usable rental space"

I don't know what text you're talking about, but I believe the answer would be C. "fifteen designs"

A can be changed to "Memorial mission" or something.

B can be changed to "Pretty structure" or something.

D can be changed to " Available leasing area".

C will always need fifteen, as that's a prime word, it's needed to explain something. If you change that word, it'll change everything else.