Final answer:
The subject of the sentence is 'It,' which is used as a placeholder to introduce the importance of attending Laura's birthday party.
The correct sentence is: b. It is important that you attend Laura's birthday party.
Step-by-step explanation:
The subject of the sentence "It is important that you attend Laura's birthday party." is 'It'. In this case, 'It' serves as a dummy subject used to introduce the sentence. The real subject, or the subject matter, is the importance of your attendance at Laura's birthday party. In English grammar, using 'It' in such a way is common for emphasizing the importance of an action or state.
The subject of the question is 'It'. In this sentence, 'It' is used as the subject pronoun referring to something that is not specified. The possessive noun 'Laura's' modifies the noun 'birthday party', indicating that the party belongs to Laura. Therefore, the subject of this question is 'It'.