= Solution
Label the external <four-momenta> by $\Psi(p_1)+\chi(p_2)\to\Psi(p_3)+\chi(p_4)$. The only <tree-level Feynman diagram> exchanges a <scalar field> particle between the transitions $\Psi(p_1)\to\chi(p_4)$ and $\chi(p_2)\to\Psi(p_3)$. Its internal <four-momentum> is $q=p_1-p_4=p_3-p_2$. Thus its <scattering amplitude>, with a consistent external-state ordering, is
$$
i\mathcal M=(-ig)^2\,[\bar u_\chi(p_4)u_\Psi(p_1)]\,
\frac{i}{(p_1-p_4)^2-m^2+i0}\,
[\bar u_\Psi(p_3)u_\chi(p_2)].
$$
It is the crossed, or $u$-channel, exchange with these labels. A direct $t$-channel <scalar field> exchange would need a flavour-preserving <Yukawa coupling>, which the <Lagrangian density> does not contain. There is no annihilation <Feynman diagram> for two incoming <fermions>.
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