Our conclusion in this paragraph is "nerve repair will be a standard medical procedure in the forseeable future". We can imagine two instances were this would not be the case:

1) "Nerve repair" does not work - and there is therefore no reason to conduct the procedure.
2) "Nerve repair" will damage the body to such a degree, that the procedure will never be conducted (because of its byproduct), regardless of whether or not it regenerates nerves in the spinal cord.

Answer (A): This states that the main purpose of this inhibitor isn't to prevent regeneration of nerves, but something else entirely. Thus, we can conjecture that the main purpose is far more important than its secondary purpose of preventing regenration. Therefore, by turning off the inhibitor, we are possibly/probably causing collateral damage to the body.

(B) This esentially states that using other nerve-growth stimulants may have the same effect as injecting the antibodies. However, the effect would be the same, and therefore does not weaken the argument.
(C) This potentially strengthens (but certainly doesn't weaken) the argument that nerve repair will be a standard medical procedure in the future, as we can conjecture that the procedure might be possible on nerves found in both the brain and the spinal cord.
(D) The paragraph clearly states that those methods do not work on the spinal cord because of that inhibitor. This doesn't add any new information.
(E) This just tells you that the procedure is time-consuming/expensive. It doesn't negate the conclusion that we will be performing this procedure. (For example, chemotherapy is both a long and expensive treatment, but is still used in combatting cancer because of its effectiveness)