The largest unit within which gene flow can readily occur is a
a. population c. genus.
b. species d. hybrid.

a. population

Explanation:

Gene flow cannot occur in hybrids because 99.9% of the time hybrids offspring are not fertile.

At genus level is not possible either because a hybrid is a cross breed at the genus level (horse-donkey).

A population is a group of species of 1 kind. A population of donkeys.

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.