Which article can complete this sentence?
I always listen to ___ radio in the morning.

The definite article, the, completes this sentence. If you do not use the definite article, it is either omitted because it is understood, or the indefinite article, a, is used. Of course, there is a different meaning to I listen to a radio in comparison with I listen to the radio. It might be difficult for a beginner in English to understand this.