The correct answer is Option B.
In the given sentence we have to choose the correct form of adverb that describes the manner in which the little boy played in the sand.
Option A- 'Quickly' is an adjective of manner but not a suitable answer because 'playing quickly' does not sound practically correct and possible.
Option C- 'Ready' is an adjective. Readily is an adverb, therefore it does not describe the action of playing and hence is the incorrect answer.
Option D- 'Excited' is also an adjective- the adverb form is'excitedly'.
Ruling out these 3 options we are now left with option B.
'Happily' is an adverb of manner and correctly describes the action of playing.
Hence Option B is the correct answer.