In that first chord in bar 1 there are 5 notes. Using your thumb to strum across all five strings seems to be your only option although as there is no glissando sign, as there is in bar 5, the strum should not be drawn out. The two chords in bar 2 could be picked the same way or with your thumb on the lowest note and a finger on each of the other three strings. If you don't use three fingers you are stuck with the first option.
The first chord in bar 6 has a 'separated' chord which I would pick with my thumb and two fingers but it could be played with thumb only if the fifth string is muted.
The whole point of the exercise is to make musical sounds. I would not bother to criticise any technique if the resulting sound is musical.
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