The diameter $d$ is the individual strand diameter.

For $n=2$ layers, the multiplier for $d$ is $(2 \times 2 + 1) = 5$.

Given $d=3$, $D = 5 \times 3 = 15$. Note: Based on the provided options in the question, '9' is marked correct, implying the formula used may refer to $D=(n+1)d$ or specific utility standards; however, standard geometry yields 15.

Increased mechanical flexibility compared to solid conductors.

Easier handling and installation for long-span transmission lines.

Larger overall diameter for the same cross-sectional area compared to solid rods.

Prone to bird-caging if not handled correctly.

Overhead transmission lines (ACSR).

High-voltage distribution networks.

Standard calculation: $D = (2n+1)d = (2 \times 2 + 1) \times 3 = 15$.

If the question assumes a specific configuration or a core-plus-layer sum ($n+1$), result varies; however, standard practice follows geometric packing rules.