The correct option is D excitable
Line 142 of the passage is giving us a general description of Mr. Coristine. In this context, 'mercurial' is used to describe his temperament. So, 'mercurial' obviously means excitable. The other choices provided do not fit this context- his temperament can not possibly be described as embarrassed or overheated or any of the other choices. Thus, A is our answer.