Question

The center of a primary eudicot root is known as the:
a. cortex
b. pericycle

Answer 1

Cortex is the outer part of the primary dicot root.
Pericycle is just next to the Endodermis the central, but according to your question Pericycle is the center of the primary dicot root

Answer 2

Cortex is the outer part of the pirmary dicot root.
Pericycle is at the center and inside the outermost part of the stele.