Concept:The correct answer is the fixed idiomatic phrase that means "many days" but is treated as a singular expression in formal English.Explanation:"Many a day" is a formal phrase equivalent to "many days" but it takes a singular verb and article-like structure.It fits naturally in sentences such as "It is many a day since we last met.""Some days" and "many days" are direct plural phrases, so they do not carry the same formal or idiomatic tone."Some day" refers to an indefinite future time, not to a span or period of time in the past.Therefore, the sentence is best completed by the idiomatic expression "many a day."Answer:C. many a day