Answer
The internal resistance of each cell is $0.33~\Omega$
Work Step by Step
Because the cells are in parallel, the total voltage in the circuit is $4\times ~1.5~V$ which is $6.0~V$.
Let $R_0$ be the internal resistance of each cell. Then the total resistance in the circuit is $4R_0+12.0~\Omega$
We can use Ohm's law to find the internal resistance of each cell.
$V = I~R$
$6.0~V = (0.45~A)(4~R_0+12.0~\Omega)$
$4~R_0 = \frac{6.0~V}{0.45~A} -12.0~\Omega$
$R_0 = \frac{6.0~V}{(4)(0.45~A)} -\frac{12.0~\Omega}{4}$
$R_0 = 0.33~\Omega$
The internal resistance of each cell is $0.33~\Omega$.