Key idea: For isomeric organic compounds, density generally decreases with increasing branching.

• More compact/branched molecules have weaker intermolecular attractions per unit surface area and usually pack less effectively in the liquid state.
• Hence, among isomers with the same molecular formula: $\text{straight chain} > \text{less branched} > \text{more branched}$ in density.

Let the given compounds be arranged from least branched to most branched as: $D,\ C,\ B,\ A$ Then their density order will be: $\rho(D) > \rho(C) > \rho(B) > \rho(A)$

Now compare with the options:

• Option A: $(D) > (C) > (B) > (A)$ ✅
• Option B: $(C) > (D) > (A) > (B)$ ❌
• Option C: $(C) > (B) > (A) > (D)$ ❌
• Option D: $(A) > (B) > (C) > (D)$ ❌

Therefore, the correct decreasing order of densities is: $\boxed{(D) > (C) > (B) > (A)}$