Option 1 -> "altar" is a noun referring to a religious table/platform, not a verb for changing behavior.
Option 2 -> "alter" is a verb meaning to change or modify something, fitting perfectly with changing behavior.
Option 3 -> "altercate" means to argue or dispute heatedly, which doesn't fit the context of improving behavior.
Option 4 -> "oust" means to expel or remove someone from a position, irrelevant to personal behavior modification.
Hence, Option 2: alter -> The sentence requires a verb that means "to change or modify." After receiving constructive feedback from his mentor, he would naturally try to alter (change) his behavior for improvement. -> correct