On the maxillary left side, there's only one posterior tooth, which is large and has a fifth cusp and three roots. this tooth is a maxillary
a. central incisor.
b. second premolar.
c. third molar.
d. first molar.

d. first molar.

Explanation:

The maxillary first molar is the type of the human tooth. It is found on the lateral side of both the maxillary second premolar and maxillary second molars. The function of this molar is like that of the other molars. It aids in grinding, chewing and mastication of food.  

It exhibit five cusps that includes, the two on the buccal side, two palatal side and the fifth one is the smallest and it can be seen on the palatal side, this is called as Cusp of Carabelli. The maxillary first molar typically have three roots.