I think we have to match the answers with the question.
1. the domain set of C = {( 2, 5), (2, 6), (2, 7)}
When we write our relation as sets of ordered pairs (x,y) like this, the domain is the set of x values and the range is the set of y values. Here the domain is correctly chosen as
Answer: {2}
2. the range set of E = {(3, 3), (4, 4), (5, 5), (6, 6)}
The range is the set of y values, so here it's
Answer: {3, 4, 5, 6}
3. the range and domain of F = x + y =10
We can write this as y=10-x. x can be any real number (that's the domain) and as x ranges over the reals, so does y, so that's the range.
Answer: domain = range = R, the set of real numbers
4. the range and domain of P = y = 3
x again can be anything but y is only ever 3.
Answer: domain = R, range = {3}