'Adverb' is a word that is used to change or qualify the meaning of an adjective, a verb, a clause or another adverb.
In the given sentence, the verb is 'walk' and 'slow' is the style of walking.
Option C - 'slowly' is an adverb which describes the verb 'walk'.
On the other hand, Option A - 'slow' is an adjective which can modify a noun, but not the verb.
Option B - 'slowed' is verb form in its past tense which can not modify another verb.
Therefore, the suitable option to complete the given sentence is : Option C - 'slowly'.
The completed sentence, thus, becomes : If you walk so slowly you will miss the bus.