Answer: 
A 
Explanation: 
Critical concentration is the concentration of tubulin dimers at which assembly is balanced with disassembly, means no net increase or decrease. 
The critical concentration is the concentration of either G-actin or the alpha, beta- tubulin complex at which the end will remain in an equilibrium state with no net increase or decrease It occurs when one end of a filament grows or increase and the other end decrease or shrink.