Which one of these engines can be completely balanced for primary forces

Question

         Given below are four “V” twin engines with different included angles between cylinder axes. Which one of these engines can be completely balanced for primary forces by attaching a single revolving mass opposite to the crank pin?                                

(a)  “V” 45⁰ twin engine

(b)   “V” 90⁰ twin engine

(c)  “V” 60⁰ twin engine

(d)  “V” 120⁰ twin engine