Answer
(a)
Work Step by Step
Note that in the definition of matrix product,
Let $A$ denote an $m$ by $r$ matrix and let $B$ denote an $r$ by $n$ matrix.
The product $AB$ is defined as the $m$ by $n$ matrix
whose entry in row $i,$ column $j$
is the product of the $i$ th row of $A$ and the $j$ th column of $B$
A is an $m$ by $r$ matrix, B is an $r$ by $n$ matrix,
r = number of columns in A = number of rows in B.
Correct choice: (a)
