Hello, again, Mr. Teo. Are you studying for the TESOL exam?
(a) "They are practising archery every weekend." As an American, I would say "practicing."
(b) "She is using lipstick," without the indefinite article "a."
(c) "A crab has a pair of pincers," because they do.
(d) You may say all of them depending on what you mean - general, specific, a small area