They change it frequently but roughly you have some requirements, and you either have to figure the solution or build the solution (i.e. hiring a candidate based on their skills and requirements, building a Lego bridge to the requirements etc)

What’s the best way to response in group exercises? I mean what things you should do in group exercise

Be the one who leads - often that means being the first one to talk/break the beginning awkward silence. There's usually some catch or something in the requirements they give you so be the one to spot it. For example the hidden catch for us in bridge building was only one person can touch one colour of the lego, so being the one to spot it shows you're observant. Also remember it doesn't matter if you pass the task or get it right, it's more about showing the instructor you're capable, so if you get the spec then be the one to say 'okay guys let's take 2 mins to read the spec then we discuss it'