An isolating switch, also known as an isolator, is a device used in electrical systems to ensure that a circuit is completely de-energized and disconnected for maintenance or inspection purposes. Below is the step-by-step explanation justifying the correct answer:

1. An isolating switch is primarily used for safety reasons to ensure that no electrical current is flowing in a particular section of an electrical system.
2. It is typically a manually operated mechanical switch. This means that a person must physically operate it to disconnect the power, ensuring that the operation is deliberate and controlled.
3. The options provided are:
   1. It is a remote-controlled hydraulic switch
   2. It is a programmable digital switch
   3. It is an automatic electronic switch
   4. It is a manually operated mechanical switch
4. Let's analyze each option:
   • Option A: "It is a remote-controlled hydraulic switch" - Isolating switches are not typically remote-controlled or hydraulic. This option does not fit the function of an isolating switch.
   • Option B: "It is a programmable digital switch" - Isolating switches are not digital or programmable. They are simple mechanical devices aimed at ensuring manual control.
   • Option C: "It is an automatic electronic switch" - Automation is generally not involved in isolating switches as they need manual confirmation for safety reasons.
   • Option D: "It is a manually operated mechanical switch" - This is consistent with the function and design of an isolating switch as it requires manual operation to ensure safety.
5. Therefore, the correct option is Option D: "It is a manually operated mechanical switch".

Conclusion: Isolating switches are designed to be manually operated to provide a visible gap in the circuit, ensuring safety during maintenance activities. This clear demarcation is critical in industries to prevent accidental contact with live parts.