= Solution
{c}
\b[TODO (source gap):] Figure 1(c) and its <four-momentum> labels are unreadable. The assertion about this individual <Feynman diagram> therefore cannot be checked against its actual integrand. In particular, one must not identify the missing panel with a convenient vanishing <seagull vertex> graph by inference.
A useful criterion, once the drawing is recovered, is that <dimensional regularization> annihilates a vector integral of the form
$$
\int\frac{d^dl}{(2\pi)^d}\,l^\mu f(l^2)=0
$$
by the change of variables $l\mapsto-l$. This would prove vanishing for every $\xi$ if all remaining factors were functions of $l^2$; a denominator involving $l+k$ must first be checked rather than discarded. Whether this criterion applies to panel (c) is precisely the unavailable information.
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