How many prime numbers less than 100 have a units digit of 1?

There are two ways to solve this problem:

1) We can write down all the prime numbers and check if they have 1 as a unit digit. Then we count them.

2) This is quicker way. We write down all the numbers that are less than 100 and that have 1 as a unit digit:
1
11
21
31
41
51
61
71
81
91
Now we check if each of them is prime number. By definition 1 is not prime number. From our list prime numbers are: 11,31,41,61,71.

So, we have total of 5 prime numbers that are less than 100 and that have 1 as a unit digit.

The correct answer will be d. -4