We can check if two fractions are equivalent by doing cross multiplication and see if the result is true.
For example, for 8/12 and 2/3 we have the following:

since we have that 24 = 24, this means that the fractions are equivalent.
For the next fraction, we have 3/4 and 20/28, then:

in this case we have that 84 = 80, which is never true, thus, the fractions are not equivalent.
For the remaining two cases,we have:

and:

therefore, only 8/12 and 2/3,
and 7/10 and 21/30
are equivalent fractions