In a sentence construction, a simple subject is a basic unit. A subject is a noun which includes the person, thing, idea or place. A sentence is not complete without a subject as it informs about what the sentence is about.
Simple subject in a sentence is a noun which is being talked about in particular. In other words, it refers to a single noun that connects or does the action.
Therefore, sentence in which underlined word is simple subject is c)The RAIN came down in the sheets.
As in the above sentence, 'the rain' is the subject who does the action of coming down solely.