- Short selling if you believe that it will go down even lower.
- Placing a long position in exchanges like Binance etc. This is you expect that the altcoin will recover.
- Buying it and then wait for it to recover.
I'm assuming that what you are saying is "all altcoins" collapsed including those large market cap coins. There are many ways to profit from these altcoins depending on which altcoin you will choose to buy. I'm always a fan of buying on the large market cap coins as they are way less risky than those with low market cap altcoins, but gives lower reward. On the other hand, maybe you should consider researching and buying those young and new projects as long as you are ready with the worst case scenario. Young and new altcoins can give you huge profits.
I think short selling will depend on what kind of coins you hold. If you are a holding a new coin and you don't know it's future then much better if sell it before more collapse are going to be recorded but if what you have is a top coin then it will be better if just hold them than selling and incur a loss as they can still recover.
For the second one that you suggest which is placing a long position, the coins will fit better here are those top coins because again like I said earlier, we already know that they can recover. Same goes to the last one in your list. There are many ways or activities that we can do to earn but I think most of them only work when the market is in good condition.