Final answer:
The statement that is not true about microtubules in a cilium or flagellum is that both inner and outer microtubules are stable.
Step-by-step explanation:
The statement that is not true about microtubules in a cilium or flagellum is d. Both inner and outer microtubules are stable. The inner microtubules are responsible for the microtubule sliding that causes bending, the outer microtubules are composed of a special "doublet" structure, and the outer microtubules carry two rows of dynein molecules. However, microtubules in a cilium or flagellum are not stable and undergo dynamic assembly and disassembly.