As you realized in the comments, your sentence uses the word as a verb.
I think the worksheet has an error (they probably meant "adjective", as in "a party leader"). Or it could just be poorly designed.
As a native speaker, I've never heard "party" used as an adverb, nor do I know of any derivatives of "party" that are adverbs. This is backed up by every regular English dictionary that I checked.
The one dictionary that does list it as an adverb is the Oxford English Dictionary (because it's a historical dictionary and contains some very rare words).
Although it's almost certainly not what the worksheet is looking for, there is one definition in the OED for "party" as an adverb (it just means "partly"), which is marked as "Now rare (only in Heraldry)". I found an example on Wikipedia:
Common partitions of the field are:
- parted (or party) per fess (halved horizontally)
- party per pale (halved vertically)
- party per bend (diagonally from upper left to lower right)
- party per bend sinister (diagonally from upper right to lower left)
- party per saltire (diagonally both ways)
- party per cross or quarterly (divided into four quarters)
- party per chevron (after the manner of a chevron)
- party per pall (divided into three parts in a Y shape)