D. A and B are obvious because sue does not equal we and dancers does not equal her. C caught me, but windowS does not equal it (which is singular). The only answer left is D.
My first guess would be that it is an independent clause. This is just a single statement.
The valve has 3 pipe connection in a T formation. The centre leg of the T is the inlet and the short arms are outlets
The appositive is usually a group of words related to a noun, usually a subject, and describes it in more detail.
The appositive here is a river that flows through Egypt.
Another way of easily spotting an appositive is by looking for commas - if it is separated from the rest of the sentence by commas, it is likely to be an appositive.