Critical concentration is Critical concentration is :

a. the concentration of tubulin dimers at which assembly is balanced with disassembly.
b. the concentration of tubulin dimers at which assembly occurs primarily.
c. the concentration of G-actin in most muscle cells.
d. required before nucleation can occur.
e. both choices A and D

Critical concentration is:  a. the concentration of tubulin dimers at which assembly is balanced with disassembly.

Explanation:

The cytoskeleton is a complex network of interconnected filaments and tubules that extends throughout the cytosol, from the nucleus to the inner surface of the plasma membrane.  Tubulin may assemble and disassemble simultaneously in a process known as treadmilling. In this process, critical concentracion is the concentration of G-actin (actin) or the alpha,beta- tubulin complex (microtubules) at which assembly is balanced with disassembly.