Answer: unless
Step-by-step explanation: a subordinated clause is a phrase that is dependent of another one. A subordinated clause doesn't make any sense by itself, its meaning relies on the principal phrase.
A subordinating conjunction is the one that introduces a subordinated clause.
In the given sentence, the principal phrase is "You cannot obtain your driver's license", this sentences has its own meaning.
The subordinated clause is "you practice driving", this phrase doesn't have a meaning of its own.
The word that introduces the subordinated clause is "unless" so that is the subordinating conjunction.