文法應用 The ideal candidate will have at least three years of experience ___ a retail store.?

「不定詞簡單式」當修飾語用時，有尚未發生的涵意，不適合本句需要（因為三年的經驗是己經發生的狀況），所以不能用to manage（＝將來管理）。
「現在分詞簡單式」的重點是做，所以可以用在正在或己發生的狀況，所以應用managing a retail store來修飾前面的名詞experience。

The question should be "why is (C) a better answer than (B)"

現在分詞片語當形容詞用來形容 experience 本來就是適當得不得了！

The present participle "(C) managing" acting as an adjective to describe "experience" is appropriate enough.Not "to" as an infinitive..
Participle in Grammar in English is a word formed from a verb, ending in --ing(=the present participle,eg:-managing), or -ed, -en, etc.(=the Past Participle).Hence eg:-Ideal canadidate will have at least three years of experience "managing" a store.