Question

Upstrokes may be used on:

All strings

One string

muiltible frets only

both a single string and all strings

Answer 1

Try it for yourself. Each option that you are given has a context of its own and we have to apply each context to properly get the correct answer.

All strings - This only depends on the kind of chord or notes that you play and specifically what tuning you are in. Naturally, if your in standard tuning and don't apply chords, then of course upstrokes would sound awful. It could be if anything mostly barred chords that sound okay when upstroked.

One String - Once again this depends on the tuning you're in. Some tunings are very forgiving and allow you to press one note and do an upstroke, but generally this is a no no in standard tuning.

Multiple frets only - If anything, I'd say that this is the option we totally and a 100% eliminate of them all. It is NOT required that only multiple frets require upstrokes because just one fret CAN be played in an upstroke and besides notes on frets, and chords in different spots can all be very versatile and workable.

Both a single string and all strings - Definitely plausible, definitely possible, and definitely a definitely. :) This can also depend on tuning as well.