背後

彼の成功の ___ には、たゆまぬ努力がある。

The correct answer is 「背後」(はいご), meaning "behind," "at the back of," or "underlying." When talking about the cause or supporting factor of someone's success, 「成功の背後には」 (behind his success) is a common expression indicating the hidden efforts or background reasons. "たゆまぬ努力" (ceaseless effort) perfectly fits this context as the underlying reason for success. 「目前」(もくぜん) means "before one's eyes" or "imminent"; this describes something immediately present, not a foundational reason. 「手前」(てまえ) means "in front of" or "this side of"; this refers to physical proximity. 「内側」(うちがわ) means "inside"; while efforts are internal, 「成功の内側には」 is not a natural idiom for underlying causes.

正解は「背後（はいご）」です。「成功の背後には」、つまり「成功の影には、目に見えないところで」たゆまぬ努力があった、という意味に合います。「目前」は目の前、「手前」はこちら側、「内側」は内部を意味し、功績の隠れた要因を表す文脈には合いません。