Condition for maximum current

An external resistance $R$ is connected to a cell of internal resistance $r$, then the current is maximum when:
a). $R<r$
b). $R=r$
c). $R>r$
d). Any other value of $R$

As $V= I (R+r)$, I think the current will be maximum when $R+r$ will be minimum. So, I think its ‘a’ because $r$ is a constant. So, the less the external resistance, the more the current. My book has answer ‘b’ and a couple of websites have the answer ‘b’ which I don’t think is right.
Can anyone help?