Simple,
it's B.
[tex] \frac{a-b}{b} = \frac{3}{7} [/tex]
That's giving you an example...
[tex] \frac{a}{b}= \frac{10}{7}
it's telling you to plug it in, simple...
[/tex]
a=10
and
b=7
[tex] \frac{10-7}{7} = \frac{3}{7} [/tex]
True, or not true?
Thus, your answer. B.