Final answer:
The compound subject of the sentence is '4 boys and 5 girls' as they are the ones performing the actions of laughing and playing.
Step-by-step explanation:
The compound subject of the sentence "Laughing and playing in the front yard were 4 boys and 5 girls." consists of multiple entities performing the actions described in the sentence. In this case, the correct answer is c) 4 boys and 5 girls, as they are the ones who are laughing and playing in the front yard, making them the subjects of the sentence. It's crucial to identify the actors in the sentence in order to determine the compound subject, ensuring that the subject matches the verb in number.