The noun “穴” (あな) means “hole.” In the context of the sentence, “庭に穴を掘って、池を作った” means “I dug a hole in the garden and made a pond.” The correct reading of “穴” in this context is “あな.”
いけを: This means “pond,” but it does not fit the context of digging a hole.
つちを: This means “soil” or “earth,” but it does not fit the context of digging a hole.
あなを: This is the correct term for “hole,” which fits the context of digging a hole in the garden.
みずを: This means “water,” but it does not fit the context of digging a hole.