The answer is b)3.
If the last digit of a number is divisible by 2, than the number is divisible by 2 (for example, in 828, the last digit is 8 which is divisible by 2 (8/2 = 4), so 828 is also divisible by 2).
If the sum of all the digits of a number is divisible by 3, then the number is divisible by 3 (for example, in 396, the sum of the digits is 18 (3+9+6=18), so 396 is also divisible by 3).
Similarly, if the sum of all the digits of a number is divisible by 9, then the number is divisible by 9.
Now, let's check it out for 10²⁵ - 7. Let's first take simple examples:
10² - 7 = 100 - 7 = 93 (The last digit is not divisible by 2. Also, the sum of the digits is 9+3=12 and it is divisible only by 3, but not by 9).
10³ - 7 = 1000 - 7 = 993 (The last digit is not divisible by 2. Also, the sum of the digits is 9+9+3=21 and it is divisible only by 3, but not by 9).
10⁴ - 7 = 10000- 7 = 993 (The last digit is not divisible by 2. Also, the sum of the digits is 9+9+9+3=30 and it is divisible only by 3, but not by 9).
....
10²⁵-7 =..... There are many zeros, but based on the previous examples, we can make a conclusion that the last digit is not divisible by 2. Also, the sum of the digitsit is divisible only by 3, but not by 9.