Quelle est la nature de l'entreprise de plomberie?
What is the nature of the business of plumbing?
Quelle fromage est-il ? isn't grammatically correct in French. Quelle heure est-il (what time is it) is correct, but fromage is a masculine word, so the pronoun is also masculine : quel. Correct questions could be : 'quel fromage est-ce' (what cheese is it?) or 'quel est ce fromage' (what is this cheese ?).