The correct answer is A) cold at idle. When a diesel engine is cold, the lube oil is more viscous (thicker), which results in higher lube oil pressure. As the engine warms up, the lube oil becomes less viscous, leading to a lower indicated lube oil pressure. At full speed, the engine components are generating more heat, further reducing the lube oil viscosity and resulting in lower indicated lube oil pressure. The other options are incorrect because: B) warm at idle would have lower lube oil pressure than when cold, C) warm at full speed would have even lower lube oil pressure, and D) warm at full speed with no fuel dilution is not the scenario that results in the highest indicated lube oil pressure.