According to Merriam-Webster, Multitude means:
"a great number of things or people"
We use multitude when refers to things or people which are in a great number, many or more than enough.
Now we look at the meaning of the options given above in Merriam-webster :
(1) Impoverished – represented by few species or individuals
(2) Handful – This means whatever amount you can hold in a hand and you can hold a few things in it, right?
(3) Acknowledged- when you acknowledge something, you accept or admit its existence.
For instance- I acknowledge the fact that there are too many bugs in the system.
(4) Plenty – When you have plenty of something, you have more than enough of something. In other words, Plenty simply refers to large amount or quantity.
For instance – I have plenty of clothes in my closet.
(5) Solitude – When you are in a state of solitude, you live alone but it is different than loneliness. In this state, you desire to live alone to give time to reflect on your thought or introspect.
Seeing all the definitions above, you might have guessed the right option. The only option that refers to a large amount is option (4) plenty.