💡 Detailed Explanation
The correct answer is 「効率」 (kōritsu), meaning 'efficiency' (output relative to input/cost). It is used in terms like 「業務の効率」 or 「エネルギー効率」. 「効果」 (kōka) means 'effect/effectiveness'. 「能率」 (nōritsu) means 'productivity/rate of work' (often used for workers' physical work rate). 「確率」 (kakuritsu) means 'probability'.