Which answer is true regarding birthday attack on digital signatures?
A birthday attacks generates a pair of colliding messages $m, m'$. Because of the way the collision is generated, you can't predict either $m$ or $m'$. If $m$ is fixed, then you need to perform a second-preimage attack instead.

The only person in this scenario who can change $m$ is $S$. $R$ and the third party only receive a fixed message $m$, and have to generate a second preimage $m'$. This is not possible using a birthday attack.

Thus, the correct answer is B. (I) only.