Boyle's Law governs the process at constant temperature

Steps:

• Pressure doubles (P₂ = 2P₁) and volume halves (V₂ = V₁/2), satisfying P₁V₁ = P₂V₂ for ideal gas.
• Mass of gas is constant in sealed syringe, so density ρ = m/V increases as V decreases.
• No information on heat exchange, but volume change matches Boyle's Law exactly, implying constant temperature.
• Thus, density increases while temperature stays the same.

Why C is correct:

• Boyle's Law (PV = constant) holds only at constant T; here, doubled P and halved V confirm T unchanged, and density rises with reduced V.

Why the others are wrong:

• A: Density cannot decrease with smaller volume; T decrease would require different PV ratio.
• B: T decrease violates Boyle's Law match; PV product would not stay constant.
• D: Density cannot remain constant with halved V; T increase would expand volume further.

Final answer: C