Right choice is (d) 2 × t50
For explanation I would say: Given, the reaction is 75 percent complete so, a = 75
We know, t = \(\frac{2.303}{k}\)log\(\frac{100}{100-a}\)
T75 = \(\frac{2.303}{k}\)log\(\frac{100}{100-75}\) = \(\frac{1.386}{k}\)
t50 = \(\frac{2.303}{k}\)log\(\frac{100}{100-50}\) = \(\frac{0.693}{k}\)
t75 = 2 × t50.