Hey,
Since the economy has a floating exchange rate the formula CA+KAFA=0 applies.
So you just need to find the totals for the CA and KAFA and then solve for the unknown.
For CA: +12,-5, NPI,-2=5+NPI
For KAFA: +45
Therefore 5+NPI+45=0, NPI+50=0, so NPI=-50 note all calcs in bn therefore the answer is -50bn so (B)