Answer is b) WXGA (1280x768)
The link to enter on that page is actually an email link, so you need to email competitions pcplus.co.uk (replace with @), with the subject as 259 Optoma competition
It doesn't specifically say what else you need to include in your email apart from your answer, so I would assume name, address, and telephone number.
I also added the following to the bottom of my email (just in case):
I do not wish for my details to be passed onto third parties.