The masculine gender of 'vixen' is

CONCEPT: English has distinct gender-specific nouns for many animals -- a general species term, a distinct masculine term, and a distinct feminine term (for example: lion/lioness, tiger/tigress, fox/vixen). For this pair, "vixen" is specifically the feminine noun for a female fox, and its masculine counterpart is "fox" itself.

APPLICATION: The question asks for the masculine counterpart of "vixen." Since "vixen" already denotes a female fox, the masculine counterpart must denote a male fox -- which is "fox," the base/masculine term in this animal's gendered-noun pair.

CROSS-CHECK: None of the other options fit this pair -- "ox" pairs with "cow" (a different, bovine gender-pair), "horse" pairs with "mare" (with "stallion" as an additional masculine-specific term, also a different pair), and "deer" is a general species name whose own gendered pair uses "buck" and "doe," not "fox"/"vixen." Only "fox" correctly completes the masculine/feminine pair with "vixen."