Answer: Option D
:
D
(a, b) R (a, b) because a + b = b + a.  So, r is reflexive.
(a, b)R (c, d) $\Rightarrow$ a+d = b+c $\Rightarrow$ c+b = d+a
$\Rightarrow$ (c,d) R (a,b)
So, R is symmetric.
(a, b) R (c, d) and (c, d) R (e, f)
$\Rightarrow$                    a + d = b + c, c + f = d + e
       Adding,         a + d + c + f = b + c + d +e
$\Rightarrow$                   a + f = b + e
$\Rightarrow$                       (a, b) R (e, f).
$\therefore$ R is transitive.