💡 Detailed Explanation
The correct choice is 'やく' (forming '役に立つ' - to be useful / helpful). The set phrase '役に立つ' (yaku ni tatsu) is a standard Japanese expression. Sentence translation: 'Knowing how to use this computer is very useful for work.' 'いみ' (meaning), 'りゆう' (reason), and 'もくてき' (purpose) do not pair with '〜に立つ'.