Ralph hides in the thicket out of sheer desperation and fear for his life. He has heard from Samneric that Roger has "sharpened a stick at both ends" and clearly mean to have a man hunt (190). Ralph "wormed his way" into the thicket, because its dense coverage will help hide and protect him from the other boys. He has a spear with him, the one that he took after he knocked the sows' skull off its spear lodged in the rock. When he is hiding from the savages, Ralph unconsciously gnaws on the spear and finds "bark in his mouth" (193).
After a close reading of the text to check for your possible choices-- Ralph does not do any of those three things: cover his face in mud, bury Piggy's glasses, or find a boar's tusk. He mostly just crouches in the thicket, paralyzed with fear and listens to the hunters moving past him. He definitely does not bury Piggy's glasses, because Jack has control of them up until the end of the novel when he still "carried the remains of a pair of spectacles at his waist" (201).