"<em>Mi hermano tiene 15 años</em>" can be literally translated to "<em>my brother has 15 years old</em>". "<em>Mi hermano"</em> is the subject, while "<em>15"</em> is the adjective modifying the noun "<em>años"</em>.
"Tener" is a verb that specifically conveys the possession of something, whether it's an object or something more abstract like age. The correct conjugation of this verb for a singular masculine person (mi hermano) is "tiene". Therefore, the only possible way to complete the sentence is with TIENE.
