Put the adverb in brackets into the correct form. Their dogs run so ___ (fast).

The sentence requires an adverb to modify the verb "run". The word "fast" can function as both an adjective and an adverb. When used as an adverb, it describes how the action is performed. The options provided are "faster", "fasty", "fastly", and "fast". "Fasty" and "fastly" are not standard English words. "Faster" is the comparative form of the adverb "fast", used when comparing two things. Since the sentence uses "so" to indicate a degree or extent, the base adverb form "fast" is the most appropriate choice to complete the sentence, implying they run to a great extent or speed. Therefore, the correct form is "fast".

Their dogs run so fast (fast).