What is the probability that a two digit number is not a prime number when a number is chosen at random?

Answer: Option B

Two digit number = 10 . . . . . 99 = 90 number
Prime number between 1 to 100 = 25
Prime number between 1 to 10 = 2, 3, 5, 7 = 4
Prime number between 1 to 99 = 25 - 4 = 21
$$\text{Probability}=\frac{90-21}{90}=\frac{69}{90}=\frac{23}{30}$$