What is a dolmen?
a. a megalithic tomb chamber built with the post-and-lintel method
b. a large stone construction
c. a tomb chamber covered with smaller rocks and dirt

C. a megalithic tomb chamber built with the post-and-lintel method

Explanation:

A dolmen is usually consisting of two or more vertical megaliths supporting a large flat horizontal capstone or "table" (post-and-lintel construction).