Answer(a) $\{2,3,4,5\}$
Explanation: Relatively prime numbers are those numbers that have only 1 as the common factor.
So, according to this definition we get to know that (2, 3), (2, 7), (3, 7), (3, 10), (4, 3), (4, 7), (5, 3), (5, 6), (5, 7) are relatively prime.
So, $R=\{(2,3),(2,7),(3,7),(3,10),(4,3),(4,7),(5,3),(5,6),(5,7)\}$.
Therefore, the Domain of $R$ is the values of $x$ or the first element of the ordered pair.
So, Domain $=\{2,3,4,5\}$