Final answer:
When 'a' and 'b' are 2x2 matrices with specified columns, 'ab' represents matrix multiplication, which is Option D.
Step-by-step explanation:
If "a" and "b" are 2x2 matrices with columns a1, a2, and b1, b2 respectively, then "ab" represents matrix multiplication. In this context, matrix multiplication is the process in which you combine two matrices to form a new matrix where the elements are formed by taking the dot products of corresponding rows from the first matrix with columns of the second matrix.
To multiply two matrices, you calculate the entry in each position of the product matrix as the dot product of the row from the first matrix and the column from the second matrix. For example, the entry in the first row and first column of the product matrix ab would be the sum of the products of the corresponding elements in the first row of 'a' and the first column of 'b'.
Therefore, Option D, Matrix multiplication, is the correct representation of the product "ab" when dealing with matrices.