Correct option (a) 500 k
Explanation:
ΔG = ΔH – TΔS
O = 4000 – T x 10
because (ΔG = 0)
T = 400 k
i.e. At 400 k, temperature reaction will be in equilibrium. But at temperature higher than 400 k the value of ΔG will be negative.
Thus at 500 k temperature reaction will be spontaneous.