10²⁹ = 2²⁹ ´ 5²⁹

10²⁹ has 30 ´ 30 = 900 factors

To be an integer multiple of 10²³, look at 10²⁹ as 10²³ ´ 10⁶

So, all factors of 10⁶ when multiplied by 10²³ are also integer multiples of 10²³.

10⁶ = 2⁶ ´ 5⁶ has 7 ´ 7 = 49 factors.

So, out of 900 factors of 10²⁹, 49 factors are integer multiples of 10²³

Probability = = =

a = 7; b = 30

difference = 23 Choice (D)