The mechanism used is oxymercuration-demercuration. In the part b its demercuration. The picture shows the structure of the product.
Work Step by Step
Look at part b in the picture. The reagent used to do demercuration of the organomercurial alcohol is $NaBH_4$. Steps: Demercuration with $NaBH_4$ replaces the mercuric fragment with hydrogen to give the alcohol.