Which of the following expressions would give the number of ways to choose a group of 7 from a group of 15?

You didn't supply any expressions, but it'd be one of these (assuming it's unique combinations, rather than the duplicate permitting permutations):

(15!) / (7! (15-7) !)

or

(15!) / (7!8!)

or

(15•14•13•12•11•10•9) / (7!)

or

(15•14•13•12•11•10•9) / (7•6•5•4•3•2)

or

32432400/5040

or

6435

depending on what it has as possible answers.