Hola Trefor

This is the test-question you came across:

 

Ella va de compras con su hija mientras Alfredo ___ en el garaje.

She goes shopping with her daughter while Alfredo works in the garage.

You need the indicative here "trabaja" because it is expressing two simultaneous actions. If we had said "as long as..." (with a conditional meaning), then you would have needed the subjunctive "trabaje". 

I hope it clarified it.

Saludos