The correct answer is option 4 i.e. slow. Explanation
As we have to find the adverb in the given sentence let us understand an 'adverb'.
An adverb is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc.
Generally, Adverbs are formed by adding (-ly) to an adjective.
Now, we have to find if any word is ending with or without (-ly) and modifying a verb or adjective.
From the given options:
'Boy' acts as a noun in the given sentence.
'Walk' acts as a verb in the given sentence.
'Now' acts as an adjective in the given sentence.
'Slow' is modifying the verb 'walk' so we can say it is an adverb.
'Slow' is used as an adjective and slowly is used as an adverb, but 'slow' can also be used as an adverb.
When an adverb does not have the usual (-ly) ending it is called a flat adverb or plain adverb and it looks the same as its adjective form.
In casual speech, informal writing (such as text messages or e-mails to a friend), and even some formal writing slow is often used as an adverb.
Hence, 'slow' is identified as an adverb in the given sentence.