The mechanism used is Oxymercuration–demercuration.
Work Step by Step
The reagents used are ethanol and $NaBH_4$. The mechanism is Oxymercuration–demercuration. Steps: 1) The arrow goes from the double bond to the Hg and back from Hg to the carbon. This electrophilic attack forms a mercurinium ion. 2) Ethanol opens the ring to give an organomercurial alcohol. It attacks on the most substituted carbon. 3) Deprotonation of the ethanol happens, leaving behind a organomercurial alcohol.