Which sentence contains a dependent clause?

i want to become an electrician.

i want to become an electrician after i graduate high school.

i want to go to college for electrical engineering.

i want to go to college and get a job.

Dependent clauses cannot stand on their own, they have to be parts of a bigger, complete, independent clause. Having that in mind, the correct answer is I want to become an electrician after I graduate high school. 
Here, the dependent clause is after I graduate high school, which doesn't sound complete, as you can see. The other options are simple sentences or contain two independent clauses, but there are no dependent ones.