D

In a diode valve, the state of saturation is achieved when a high plate voltage is applied along with a sufficient filament current.
Step 1: Understand the function of the diode valve.
A diode valve allows current to flow in one direction and has a cathode and anode. The cathode must be heated (using filament current) to emit electrons.
Step 2: Identify conditions for saturation.
For saturation, both high plate voltage and high filament current are essential to ensure that a large number of electrons are emitted and subsequently collected by the plate.
Step 3: Analyze the options.
- Option A: High plate voltage and high filament current is true for saturation but does not specify the filament clearly.
- Option B: Low filament current is insufficient to achieve saturation.
- Option C: Low plate voltage does not favor saturation as it doesn't provide enough potential difference.
- Option D: High filament current ensures enough electron emission, and high plate voltage allows for maximum current flow. Hence, this is the condition that achieves saturation effectively.
Therefore, the correct option is D.