Answer
$a.$
columns ... rows
$b.$
$(ii)$ and $(iii)$
Work Step by Step
If $A=\left[a_{ij}\right]$ is an $m\times\fbox{$n$}$ matrix and $B=\left[b_{ij}\right]$ an $\fbox{$n$}\times k$ matrix,
then their product is the $m\times k$ matrix $C=\left[c_{ij}\right]$
where $c_{i}$ is the inner product of the ith row of $A$ and the $j$ th column of $B$.
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$a.$
n, the number of columns in A = number of rows in B
$b.$
$(i)\quad$ (3$\times\fbox{$3$}$ ) times ($\fbox{$4$}\times$3) is not defined
$(ii)\quad$(4$\times\fbox{$3$}$ ) times ($\fbox{$3$}\times$3) is defined. The product is a 4$\times$3 matrix.
$(iii)\quad$(3$\times\fbox{$3$}$ ) times ($\fbox{$3$}\times$3) is defined. The product is a 3$\times$3 matrix.
$(iv)\quad$(4$\times\fbox{$3$}$ ) times ($\fbox{$4$}\times$3) is not defined
