I think it should be option a)
All query $1,3,4$ are equivalent but $2$ is not equivalent.
Query $2$ should be
$\prod \text{Room no,hotel no,type,price}(\sigma \text{type='single' } \wedge \text{price=200 })$
When you don't use project and do only select, by default all the columns are returned, thus making Query 2 same as Query 1. Eg:
@arvin Sure, here you go. http://infolab.stanford.edu/~ullman/fcdb/aut07/slides/ra.pdf From Jeffrey Ullman's DBMS slides.
You can find all of them here: http://infolab.stanford.edu/~ullman/fcdb/aut07/
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