Find a common denominator. The easiest way to do this is by multiplying 6 Ă— 8 (=48.)
So now we have to make the numerators match the denominators, so multiply 6 by 8 for the first fraction, and 6 by 6 for the second frcation.
The first fraction now becomes:
whereas the second fraction now becomes:

Now it should be easier to see, so you tell me, which one is bigger?

or
?
Obviously it's the first one, as 48 is a bigger number than 36, so the correct answer is 6/6.
(Side note: whenever the numerator is the same as the denominator, it means that it is just 1 in decimal. Therefore, any other normal fraction (e.g. 6/8) would be smaller.)