1. The problem asks about the condition for matrix multiplication $A \times B$ to be defined. 2. Let matrix $A$ have dimensions $m \times n$ (rows by columns) and matrix $B$ have dimensions $p \times q$. 3. Matrix multiplication $A \times B$ is possible only if the number of columns of $A$ equals the number of rows of $B$, i.e., $n = p$. 4. This is because each element in the resulting matrix is formed by the dot product of a row of $A$ and a column of $B$, which requires these dimensions to match. 5. Evaluating each option: a. "number of rows in A must equal to columns in B" means $m = q$, which is not needed. b. "number of columns in A must equal to columns in B" means $n = q$, which is not correct. c. "number of rows in A must equal to rows in B" means $m = p$, which is not the condition. d. "number of columns in A must equal to rows in B" means $n = p$, which is the correct condition. Final answer: Option d is correct. The number of columns in $A$ must equal the number of rows in $B$ for $A \times B$ to be possible.