The piano and the pipe organ have/are to be tuned for the show. Here both seems fine to me but the choice question wants me to choose one. Is there any prominence for one answer. Please help.
Given that it is a necessity that both instruments sound true during the show, we could say that the sentence is trying to express the urgency of tuning the instruments. Therefore, it can be that "have to be" is most appropriate in this context, although both versions are grammatically acceptable. As Dan added, context is lacking to precisely pin down the correct one.
Looking at difference in meaning between have to be and are to be:
have to be expresses a prerequisite for the show. If the instruments are not tuned, the show can't start. It is therefore closely related to "the instruments must be tuned" (but not the same)
are to be expresses a lower sense of necessity that the instruments are tuned, and rather indicates that doing so is general procedure. It has less emphasis on the "must", and instead it seems to convey that "this is always the case so the instruments will also be tuned this time". Like Grizzly suggested, it is expressing the expectation that the instruments are tuned.
In my mind, the phrase "are to be" conveys the sense that actual arrangements have been made to tune the piano and organ. The phrase "have to be" conveys only the necessity of tuning them without any indication that actual arrangements have been made.
"are to be" is usually pointing to the nature of the thing. for example: task is a piece of work to be done. which means that task in terms of being task has to be done. while "has to be" is more general. - so i think you should use "have to" in your example.
Are to be expresses the idea of something that is supposed to be, expected to be like that. Example: If endangered species are to be saved rain forests must be protected.