3. mammals lack the enzymes for the net conversion of acetyl-coa into oxaloacetate or any other citric acid cycle intermediate, but if oxaloacetate (or some other citric acid cycle intermediate) is drawn off for biosynthesis, it must be replenished. how is oxaloacetate replenished? under what cellular conditions would this enzymatic step be favored?

-Oxaloacetate is replenished by enzyme pyruvate carboxylase.

-This enzymatic reaction is favored when there is an increased demand of energy required by the cell.

Explanation:

Citric acid cycle is a major pathway for the ATP production. Oxaloacetate plays an important role in citric acid cycle as it starts with transfer of acetyl group of acetyl CoA to oxaloacetate with formation of citrate.

As mammals lack enzyme to convert acetyl-CoA into oxaloacetate, in mammals oxaloacetate is replenished by enzyme pyruvate carboxylase that performs carboxylation of pyruvate to form oxaloacetate.

Activity of this enzymatic reaction increases by high energy demand of cell,  and increased level of acetyl CoA as it shows increased demand of oxaloacetate to perform citric acid cycle for energy production.

the answer to your question is b