Sabit was absent from class yesterday______ he felt sick.

The right answer will be: Because. 

Because: meaning and use
- Because introduces clauses of cause and reason.
- It is a subordinating conjunction.
- This means that the clause it introduces is a subordinate clause, which needs a main clause to make it complete.
- We use a comma when the subordinate clause comes before the main clause.
Example:  Everyone left early because Rahim and Karim had an argument.

Source: Cambridge Dictionary Grammar Reference.