Modern tetrapods are mostly pentadactyl because: A. They evolved from ancestors with five digits B. They need five digits for walking only C. It increases their body temperature D. It is required for reproduction

Modern tetrapods are mostly pentadactyl because:
The pentadactyl limb (having five digits) is a homologous structure found in most tetrapods, meaning it was inherited from a common ancestor that possessed this five-digit limb structure. Evolutionary changes have modified this basic plan for various functions, but the underlying five-digit pattern remains a common ancestral trait.

The final answer is $\boxed{\text{A. They evolved from ancestors with five digits}}$.