찹쌀 is "sticky rice or sweet rice" in English and 떡 is "rice cake".
So this rice cake is sticky. of course.
합격하다[hapgyeokhada] - to pass (a test)
or We say "붙다"[butda] - literally to stick (to) <It means "to pass (a test) in Korean>
(시험에) 합격하다 = (시험에) 붇다 / 시험[siheom]: a test
Wish someone could stick right to 합격[hapgyeok] / Wish someone the best of luck
This is the origin.
엿[yeot]: taffy could be instead of 찹쌀떡. It is the same origin as 찹쌀떡.