 Solution

Choose three distinct projective points $A,B,C$. Their images under the injective map $\tau$ are distinct. Sharp triple transitivity gives a projective transformation $S$ agreeing with $\tau$ on these three points. The map $\sigma =S^{-1}\circ \tau$ fixes $A,B,C$ and preserves their cross-ratios.

For any other projective point $P$, invariance givesLet $T$ normalize $(A,B,C)$ to $(\infty ,0,1)$. By part (b), the left side is $T(\sigma (P))$ and the right side is $T(P)$. Since $T$ is a bijection, $\sigma (P)=P$. It fixes the original three points too, so $\sigma$ is the identity map everywhere. ThereforeNo continuity or prior surjectivity assumption was needed. This is the rigidity assertion that cross-ratio preserving maps are projective.