The answer is C; I made my decision and stand by it.
The best restatement for the sentence "Right or wrong, I assumed this ground, and now avow it." is C because it states the position taken by the speaker who faces a predicament, which might open chances for accusations, yet declares, "I made my decision (I assumed this ground) and stand by it (and now avow it)" Even if others may consider wrong, this is my stance.