You need to remember two definitions.
First, the dimensions of a matrix are the number of rows by the number of columns. If a matrix has a row and b columns, it's an a*b matrix.
Second, before computing the product of two given matrices, you need to check an important condition.
If a matrix A has i row and j columns. And a matrix B has k row and l columns we can not apply the product AB only if j = k. In other words the numbers of columns of A (j) must be equal to the number of rows of B (k).
If this condition is checked, than the new matrix will be of dimensions : ixl
In our case i = 2, j = 3, k = 3, l = 1
So the dimensions will be 2x1 so the answer is C.
Good luck
