= Solution
The intermediate <photon> carries <spin> $J=1$. In the massless limit, a back-to-back <fermion>-<antifermion> pair coupled to its vector current has opposite <helicities>, so the initial and final projections along their respective axes are $m,m'=\pm1$. A rotation between these axes contributes the <Wigner D-matrix> elements $d^1_{m'm}(\theta)$. The relevant elements have magnitudes $(1+\cos\theta)/2$ and $(1-\cos\theta)/2$. Summing the four squared <helicity amplitudes> gives the angular factor
$$
2\left(\frac{1+\cos\theta}{2}\right)^2+
2\left(\frac{1-\cos\theta}{2}\right)^2=1+\cos^2\theta.
$$
The common coupling and normalization factors recover the result in <1/d>. The $m=0$ terms do not contribute: the massless external vector currents eliminate the corresponding <helicity> configurations.
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