Final answer:
The term for a woman in an extramarital sexual relationship with a married man is 'A mistress'. This describes someone who may have financial support from the man but is not married to him.
Step-by-step explanation:
The term that refers to a woman having an extramarital sexual relationship, especially with a married man, is B) A mistress. This term historically describes a woman who is in a long-term sexual relationship with, and is financially supported by, a man who is married to someone else. It is important to note that the term carries a range of connotations and can be viewed differently in various cultural and historical contexts. The provided reference to Marcellus in the Digest suggests disapproval of such relationships in historical legal contexts.