D. S=-10, T=-2 is the answer.
While we check all the given options, we will see that no option other then option D gives us the equal value of difference between R and S, and R and T; which doesn't makes the given value of S and T in option(s) A, B and C equidistant from R as given in the question. Thus, it gives us option D as the answer.
Mathematically-
Considering the option D only,
R - S = -6 - (-10) = -6 + 10 = 4
and, T - R = -2 - (-6) = -2 + 6 = 4
thus, R - S = T - R