What is the participle in this sentence?

the laughing child looks happy, but she was sulking this morning.

a.
happy

b.
looks

c.
was sulking

d.
laughing

The correct answer is D, 'laughing'. 
A cannot be correct because that is an adjective.
B is a verb.
C is also a verb used in past progressive tense. Since both words are given here, it cannot be the correct answer. If it said only 'sulking', then it would be a participle. This way this is not a correct answer.