Which cassette?

[Blackandblue]

Please forgive my ignorance but what cassette should I buy for my new rear wheel (to replace the one that was nicked).

The bike has/had a Shimano 24 speed Alivio gear set up. I presume I need a Shimano cassette block but is it 8 speed, 9 speed, 10 speed....?

 

[dodgy]

8 speed (based on 24 divided by 3).

 

[Blackandblue]

Thank you!