Combination of plots which does NOT represent isothermal expansion of an ideal gas is:

🧠 Isothermal expansion of ideal gas: $T = \text{const}$, $PV = \text{const}$, $\Delta U = 0$ Any plot that violates these three conditions does NOT represent isothermal expansion.

🗺️ What each plot must show For isothermal expansion:

• $P$–$V$ graph: hyperbola ($P \propto 1/V$) — $P$ decreases as $V$ increases ✓
• $P$–$T$ graph: horizontal line ($T$ = constant) ✓
• $V$–$T$ graph: vertical line ($T$ = constant) ✓
• $\Delta U$ vs anything: zero (for ideal gas, isothermal) ✓
• $P$–$V$ product: constant (Boyle's Law) ✓

Plots B and D in the figure show relationships that contradict isothermal conditions — for example, showing $T$ changing or $PV$ not constant — making them the incorrect pair.

⚠️ Trap: An adiabatic expansion also has decreasing pressure with increasing volume, but the curve is steeper than an isotherm. Don't confuse the two on a $P$–$V$ diagram.

$\boxed{\text{Answer: (c)}}$