Simplifying a MUX's truth table
The Select signals dictate which input is reflected on the output of the multiplexer.  The don't cares show that the output is not affected by those inputs.  If you were to increase the number of inputs for the multiplexer, you have to scale your number of Select signals accordingly.  The SOP would be dictated based on what input you want on the output, based on your Select signals.

The diagram seems to be a bit misleading, it would make more sense to me if the output showed:


Output Q = A, when inputs b = 0 and a = 0
Output Q = B, when inputs b = 0 and a = 1
Output Q = C, when inputs b = 1 and a = 0
Output Q = D, when inputs b = 1 and a = 1