(a) set up is best. (b) camped is okay. Also, made is okay. (c) I think you want to say, "They had reached the jungle on a sunny day, so they set upa camp to take a rest." "Built up" makes sense and it is understandable why you would use it, but "set up" or would be the more common and accepted way to say it. (d) Again, while "built" and "built up