Sooo.....the poll was a bit of a trick question. I'm a lame-ass like that...
It's a easy answer, vocals are most used. Ask any parent/uncle/auntie!
If I had asked what is the most learned instrument, then we'd have a interesting comparison between...
Piano and guitar - little in the way of surprises there, both can be used in solo lead and accompaniment contexts. Interesting thing about a harmonica - it's similar - I forgot that you can play chords on a harmonica.
Depending on who you believe it's either guitar or piano at No.1 - I havn't seen really comprehensive stats to support guitar > piano . But, from what I read, I'd guesstimate that Piano is at least twice as learnt (likely school cirriculum's have a large part in this). Also didn't see any useful stats on how long they learnt or level of competency achieved.
I did think recorder would feature high (it does, perhaps higher than guitar) - again school has a hand in this. And it's a cheap instrument to start on.
According to the fairly decent stats collected by The Associated Board of the Royal Schools of Music (an examinations board and registered charity based in the UK) - Alas, the sample sizes are fairly small (1123 in 2014) and a small age range (5-14 y/o). https://gb.abrsm.org/en/making-music/4-the-statistics/
I'd guess the numbers could be extrapolated to western schools (UK, USA/Canada, Oz, RSA, W.Europe), though I'm curious about Asian, African, E.Europe and South American regions.
I'd count keyboard + piano (43%) and electric guitar + bass guitar + classical guitar (27%) as the top 2 though.