= Solution
For any $R>0$, set $U_R(y)=u(Ry)$ and $a_R^{ij}(y)=a^{ij}(Ry)$. These coefficients retain the same bounds and ellipticity, and a change of variables in the <weak formulation> shows that $U_R$ is a nonnegative <weak subsolution> on $B_1$. Part (a) gives the scale-correct estimate
$$
\boxed{\|u\|_{L^\infty(B_{R/2})}
\leq CR^{-n/p}\|u\|_{L^p(B_R)}
\leq CR^{-n/p}\|u\|_{L^p(\mathbb R^n)}.}
$$
The constant is independent of $R$. Fix a <Euclidean ball> $B_s$ and let $R\to\infty$ with $R>2s$. The right side tends to zero, so $u=0$ almost everywhere on $B_s$. Since $s$ is arbitrary, the <Liouville theorem for integrable nonnegative elliptic subsolutions> proves
$$
\boxed{u=0\text{ almost everywhere on }\mathbb R^n.}
$$
This is $u\equiv0$ as a <Sobolev space> element. Any continuous representative is identically zero pointwise; an arbitrary measurable representative can be modified at isolated points without changing the weak equation. Integrability and the explicit scaling force the conclusion.
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