What is the name of the data type for a transformation matrix in high-level shading language (HLSL)?

What is the name of the data type for a transformation matrix in high-level shading language
(HLSL)?

A.
sampler

B.
int

C.
fioat4x4

D.
double4

Explanation:
http://www.catalinzima.com/xna/tutorials/crash-course-in-hlsl/