= Solution
Encode each $k$-tuple of nonnegative integers by one nonnegative integer using a computable <pairing function>. Define a unary partial procedure $P$ on input $m$ as follows: use <dovetailing> on the computations of $f_{n,k}$ over all encoded $k$-tuples, and halt as soon as one of them halts with output $m$. Then
$$
P(m)\downarrow
\quad\Longleftrightarrow\quad
\exists(m_1,\ldots,m_k)\in\mathbb N_0^k:
f_{n,k}(m_1,\ldots,m_k)=m.
$$
By the <Church–Turing thesis>, this effective procedure is implemented by a register machine with some fixed code $j$, so
$$
E=\{m\in\mathbb N_0:f_{j,1}(m)\downarrow\}.
$$
\b[Thus $E$ is the domain of a <partial computable function> and is recursively enumerable.]
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