Bunuel wrote:

What is the units digit of positive integer n?

(1) When dividing n by 11, the remainder is 3.

(2) When dividing n by 10, the remainder is 6.

Given n is a positive integer

From statement 1

n=11k+3

i.e., n= 3, 14, 25, 36, 47, 58, 69, 80, 91, 102, 113, 124, 135, 146..

As you can see units digit keeps varying

Hence insufficient

From statement 2

n=10k+6

i.e., n= 6, 16, 26, 36, 46, 56, 66, 76, 86, 96, 106, 116, 126, 136, 146...

Combining we see that

n=36, 146, ....

units digit= 6

Sufficient

Hence C