His biographers discovered to their surprise that he was an atheist to the backbone. A. completely B. by and large C. by birth D. to some extent

The correct answer is A. completely.

The phrase “to the backbone” means “completely” or “in every way.” In this case, the biographers were surprised to discover that the subject of their biography was an atheist, meaning that he did not believe in the existence of any gods.

The other options are incorrect because they do not accurately reflect the meaning of the phrase “to the backbone.” Option B, “by and large,” means “in general” or “for the most part.” Option C, “by birth,” means “from the time of one’s birth.” Option D, “to some extent,” means “in part” or “to a degree.”