A.works
the competition 是主语,works是谓语动词.
work to是动词+介词的结构,是“根据...行事”的意思.
from experienced staff members是介词短语当形容词用,作为the competition 的定语.
some of whom are higher in rank是非限定性的定语从句,是 members的定语.
这句主要成分这样the competition works to.
翻译：这项比赛是根据.行事.