Question

look at this thesaurus entry:
essential
part of speech: adjective
definition: crucial
synonyms: critical, fundamental, main, necessary, required
which are essential for skiing?
ski goggles
skis

Explanation:

To determine what's essential for skiing, we use the definition of "essential" (crucial/necessary) from the thesaurus. Skiing requires equipment to slide on snow, and skis are fundamental for the activity itself—without skis, one can't ski. Ski goggles are important but not as core as skis for the basic act of skiing.

Answer:

skis