My wife and I want to go to Australia. She wants to pursue her diploma in hotel management and so we are on the verge of writing our SOP. Should I state that she is single or married, as we heard that there is a chance of cancellation of the visa if the diploma student is declared married?
You must tell the truth, because if you don't, any visas issued can be cancelled. When you fill the visa application form, there'll usually be a box that goes "I declare that all information contained in this application are true to the best of my knowledge. I understand that false declaration may lead to consequences up to and including the cancellation of your visa". There is probably a similar box in the university's application form. You are supposed to tick it to say you understand.
If you lie about your marital status and are discovered, then you can expect some pretty severe legal consequences, and you can expect to be expelled from the university.