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Let $A$ be defined as

$$A:=_C^1 leq 1.$$

I have shown that the set is precompact. But is $A$ a complete set? Or an other question: Is $A$ a closed set?

The set is closed since it is the inverse image of $[0,1]$ with respect to the continuous map $lVertcdotrVert_C^1$.

But it cannot possibly be compact because the only normed spaces with compact closed unit balls are the finite-dimensional ones. And your space isn't.

I don't know which norm the norm $lVertcdotrVert_C^1$ is, but I suppose that it is such that $C^1bigl([0,1],mathbb Rbigr)$ is complete. If that's so, then $A$ is complete (since it is closed). Therefore, it cannot possibly be precompact, since compact $iff$ precompact and complete.

The answer is NO.

Consider the sequence
$$
f_n(x)=frac1n+1sin(nx),,,ninmathbb N.
$$
Then
$$
|,f_n|_C^1= max |,f_n|+ max |,f_n'|=frac1n+1+fracnn+1=1.
$$
If $,f_n$ possessed a converging subsequence $,,f_n_k$, in the $C^1-$sense, with limit $,f,,$ then $,,f_n_k$ would also converge to $f$ in the uniform sense. But, $,f_n_k$
converges uniformly to $fequiv 0$. Nevertheless, $,,f_n_k$ DOES NOT converge in the $C^1-$sense to $0$, since $,|,f_n_k|_C^1=1$.