= Solution
This labelled part supplies a definition rather than a separate task. Its notation is interpreted as an incoherent charge-squared sum: the <photon> couples to a <quark> with <electric charge> $e e_q$, and the squared <scattering amplitude> weights its contribution by $e_q^2$. To make the <antiquark> convention explicit, let
$$
U=u^p+\bar u^p,\qquad D=d^p+\bar d^p,\qquad S=s^p+\bar s^p.
$$
Then, restricting the displayed example to three active light <quark flavors>, the <deep-inelastic structure function> is
$$
\boxed{F_2^p=x\left[\frac49U+\frac19(D+S)\right].}
$$
Thus the source's sum must either include <antiquarks> as separate <partons> or define each $q^p$ to include its antiquark distribution. It must not silently omit the <sea quarks>, especially in the small-$x$ limit used below.
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