(A) "As a ritual, Mona ______ every day" → (III) sings - Habitual/routine actions require simple present tense.
(B) "Mona will be ______" → (I) singing - Future continuous tense needs the present participle (-ing form).
(C) "Mona ______ since she was five years old" → (IV) has been singing - "Since" + past time point requires present perfect continuous to show an action continuing from past to present.
(D) "Mona ______ so many times that she knew" → (II) had sung - Past perfect tense is needed because the singing happened before another past action (knew).
Hence, Option 4: (A) - (III), (B) - (I), (C) - (IV), (D) - (II) → This correctly applies the rules of tense usage: simple present for habits, present participle for continuous forms, present perfect continuous for actions spanning from past to present, and past perfect for actions completed before another past event → correct