= Solution
A <diffeomorphism> $\phi:M\to M$ is a smooth bijection with smooth inverse. It acts covariantly on a differential form by the <pullback of a differential form>:
$$
\omega\longmapsto\phi^*\omega.
$$
For vector fields $X_0,\ldots,X_p$,
$$
(\phi^*\omega)_x(X_1,\ldots,X_p)
=\omega_{\phi(x)}(d\phi_xX_1,\ldots,d\phi_xX_p).
$$
The coordinate definition of the <exterior derivative>, or its characterization as the unique natural graded derivation extending the differential of functions, gives
$$
d(\phi^*\omega)=\phi^*(d\omega).
$$
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