= Solution
Let $M$ be a Markov blanket of the treatment $A$ inside $(A,X)$, and write the remaining adjustment variables as $N=X\setminus M$. By definition,
$$
A\perp N\mid M.
$$
The sufficiency of $X=(M,N)$ gives
$$
A\perp Y(a)\mid(M,N).
$$
Apply the <contraction axiom for conditional independence> with first variable $A$, second variable $N$, third variable $Y(a)$, and conditioning variable $M$. It gives
$$
A\perp(N,Y(a))\mid M.
$$
The <decomposition axiom for conditional independence> then yields $A\perp Y(a)\mid M$. Hence every Markov blanket of $A$ in $(A,X)$ is itself a <sufficient adjustment set>.
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