Ez egy szék would be better translation imo, it sounds very unnatural.
I beg to differ.
It appear Ez a szek would be a better translation.
Why? That would mean "This is the chair" (or "this chair" if you don't mean it as a whole sentence), but the English has "a chair" and not "the chair".
I typed "this chair", but I guess it depends in what situation it is said it would sound weird
Questionable. That translates as "thi chair"