He lost his money.His money was -----.(a)losing (b)missing (

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Lost the money meaning it happened in the past,so it is not losing.He did not continuously lost the money.
However,since the money is gone,that's why it is missing.
The following sentence can use losing -
He has LOST a lot of money in the casino and he is still losing heavily.
It means he is still gambling and still losing his money in the games.
So,don't gamble :-)
Good luck