= Solution
The <propagator> carries a real-species label rather than a charge-flow arrow:
$$
\boxed{D_{ab}(p)=\frac{i\delta_{ab}}{p^2-m_0^2+i0}.}
$$
Differentiating the interaction with respect to its four external real <fields> gives the compact <tensor> rule
$$
\boxed{v_{abcd}=-\frac{i\lambda_0}{2}
(\delta_{ab}\delta_{cd}+\delta_{ac}\delta_{bd}+\delta_{ad}\delta_{bc}).}
$$
Thus four $\phi_1$ legs or four $\phi_2$ legs give $-3i\lambda_0/2$, while two of each give $-i\lambda_0/2$. Vertices with an odd number of either species vanish. For example, $4!(\lambda_0/16)=3\lambda_0/2$, whereas $2!2!(2\lambda_0/16)=\lambda_0/2$. These factors already account for assignments at one vertex; graph <symmetry factors> must still be included separately. <Momentum> conservation and loop measures are the same as in part a.
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