The value of any number multiplied by 1 stays exactly the same, right? Well, as it turns out, 1 can be written as the fraction 7/7, or the fraction 8/8, or 9/9, 10/10, 11/11... I could go on and on to infinity, but there's a pattern there. 1 simply means "1 whole," or "all of it." "All of it" looks different in different denominators, but the core idea is the same: if we split something into n pieces, "all of it" means we have all n of those pieces. The numerator and denominator will always been the same, no matter how we want to represent 1.
What does this have to do with our problem? Well, we don't want to change the value of our fraction, we just want to change its label. So what we're going to do is multiply it by 1, but we're going to make sure to pick the right label for that 1.
7/12 x 1 = 7/12. This will be true no matter what. Let's see which of these options actually fit the bill:
Can we get this fraction by multiplying 7/12 from some form of 1? Well, 14 = 7 x 2, so let's see what we get if we pick the form 1 = 2/2:
Nope, not quite. 14/28 is not equivalent to 7/12.
What about 21/36? 21 = 7 x 3, so let's give the form 1 = 3/3 a shot:
There we go! All we did there was relabel 7/12 by multiplying by form of 1. Since we never changed its value, we can stop our search here and conclude that 21/36 is equivalent to 7/12.