Sara eats very

a-slow
b-slowly

Please the answer and explain

Sara eats very slowly.

Explanation:

'Slowly,' an adverb, is needed here as it modifies the verb 'eat.' It tells you how she eats.

'Slow' is an adjective, and as such, would not make grammatical sense in this context  unless you were to recast the sentence to:  "She is a slow eater."  (in which 'slow' describes the noun 'eater.')