The question is about field failure protection in an alternator. Let's break down the options to understand what this protection entails and determine the correct answer.

**Understanding Field Failure Protection**

An alternator, or synchronous generator, has a field winding that requires excitation current to produce a magnetic field. The field failure, or loss of excitation, means that the rotor of the alternator loses its magnetic field, which may lead to unstable operation or even damage to the alternator if not addressed promptly.

**Correct Option Explanation**

• **Loss of excitation current in the synchronous generator field winding:** This option matches the concept of field failure protection directly. The protection scheme detects when the excitation current is lost and takes corrective actions such as shutting down the alternator to prevent damage.

**Eliminating Incorrect Options**

• **Variation in illumination intensity inside generator control panels:** Illumination intensity is unrelated to field excitation. It is more of an operational visibility concern.
• **Reduction in paint thickness of alternator outer protective enclosure:** This is a mechanical or aesthetic feature and not directly related to electrical field failure.
• **Change in colour of lubricating oil storage tank surface coating:** This option is irrelevant to field failure, as it pertains to storage and maintenance rather than electrical function.

**Conclusion**

The correct answer is: Loss of excitation current in the synchronous generator field winding, as it directly pertains to what field failure protection aims to detect.