Answer:
The answer is actually the second option:
It is used as an adjective to describe the noun girls.
Step-by-step explanation:
In "Shrieking from the tenth row, the girls were no doubt his biggest fans," the phrase "shrieking from the tenth row" is what we call a participial phrase. It is constituted of a participle and its complements or modifiers. A participial phrase functions as an adjective, modifying a noun in the main clause. In this case, it is modifying the noun "girls", which is the subject of the main clause. The participial phrase is giving us further information about the girls, allowing us to understand the affirmation that they were the "biggest fans" with more detail. That is what an adjective does. It offers further information about a noun, modifying it.