One set of keys, not one key.
I would accept numbers 1 and 6 only. I would also recommend that if you are giving instructions you make them extra clear. Say:
Press the enter and alt keys simultaneously.
Press the enter key and the alt key at the same time.
Press the enter and alt keys together.
Press the enter key followed by the alt key. (if you want them not together)
If have instructions that say "Enter+Alt" or "Press Enter+Alt" it is shorthand to indicate that you should press the two keys together. "Ctrl+Alt+Del" is a common shorthand for pressing the keys control, alt and delete simultaneously. Verbally, we would say "Press control alt delete."
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