Knauft's study of sister-exchange marriage among the gebusi showed what percentage of first marriages were sister exchanges?

I believe the answer is: Just over 50%

in Gebusi culture, the sister exchange often used to for the benefit of the tribe's interest. For example, they often use the sister exchange to build a peaceful treaty with neighboring tribes or as a form of exchange for a certain resources (such as foods) 

Ithink the answer you are looking for is (b).