= Solution
By part (a), the <Paris model> $M$ is well-founded. Let $\pi$ be an <automorphism of a first-order structure> of $(M,\in^M)$. External <epsilon induction> shows that every element is fixed: if $\pi(y)=y$ for every $y\in^M x$, then preservation of membership and extensionality give
$$
\pi(x)=\{\pi(y):y\in^M x\}=x.
$$
Hence $\pi$ is the <identity function>. This is <Paris models are rigid when their complete theory has a well-founded model>, so \b[every Paris model of $T$ is a rigid first-order structure.]
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