The active site of an enzyme
a. the region of an enzyme that attaches to a substrate..
b. the region of a substrate that is changed by an enzyme.
c. the region of a product that detaches from the enzyme.
d. the highly changeable portion of an enzyme that adapts to fit the substrates of various reactions.

d. the highly changeable portion of an enzyme that adapts to fit the substrates of various reactions

Explanation;

Enzymes are globular protein of tertiary structure. They function in speeding up the rate of organic chemical reactions in living organisms.  Each enzyme has a unique binding site, which the m (substrate) must fuse or bind with precisely for the (substrate) to be converted to Products. The substrate must adjust to fit in to the active site; to form first the Enzyme-substrate complex; before broken down to products. This is called Induced fit hypothesis.  

If the three dimensional structure of tertiary structure of protein is destroyed, this affects the active site and the efficiency of the ENZYME. Therefore substrate binding with active site is affected, and no reaction take place.