In the context of paraphrasing, the words that cannot be changed due to their specific meanings and importance in the passage are:
C. “fifteen designs”
This phrase includes a specific quantity ("fifteen") which conveys a precise detail that cannot be altered without changing the intended meaning. While terms like "monumental task," "handsome building," and "usable rental space" are important phrases, they can be paraphrased into synonyms or similar expressions without losing the overall meaning.