Question

specific part of an enzyme where a substrate fits
sucrase
active site
how do enzymes speed up chemical reactions?
acid

Explanation:

The question is about the specific part of an enzyme where a substrate fits. Sucrase is an enzyme, not the part where substrate fits. "How do enzymes speed up chemical reactions?" is a question, not the answer. Acid is not related to the enzyme - substrate binding site. The active site is defined as the specific region of an enzyme where the substrate binds and the catalytic reaction occurs.

Answer:

B. Active Site