Step 1: Use the idea of a scale factor instead of writing the full ratio.
Since the two triangles are similar, one is just a scaled up or scaled down copy of the other. If we find the single number that scales one triangle into the other, we can multiply it directly into any side.
Step 2: Find the scale factor from the given relation.
We are told $2AB = DE$, which we can rewrite as:
\[ DE = 2 \times AB \]
This tells us every side of $\Delta DEF$ is exactly 2 times the matching side of $\Delta ABC$, because that is what similarity with a scale factor means, once one pair of matching sides has ratio 2, every pair of matching sides has that same ratio 2. So the scale factor going from $\Delta ABC$ to $\Delta DEF$ is $k = 2$.
Step 3: Match up the corresponding sides.
Since $\Delta ABC \sim \Delta DEF$, vertex $A$ matches vertex $D$, vertex $B$ matches vertex $E$, and vertex $C$ matches vertex $F$. So the side $BC$ (between the second and third vertices of the first triangle) matches the side $EF$ (between the second and third vertices of the second triangle).
Step 4: Apply the scale factor directly to the matching side.
We are given $BC = 8\text{ cm}$. Since $EF$ is the scaled up copy of $BC$ with scale factor 2:
\[ EF = k \times BC = 2 \times 8 = 16\text{ cm} \]
Step 5: Final answer.
$EF = 16\text{ cm}$, which is option (D).
\[ \boxed{EF = 16\text{ cm}} \]