Which type of heat detector responds to the movement of flame or its radiant energy?

The correct answer is the flame detector. This type of heat detector is specifically designed to respond to the presence of flames by sensing the infrared or ultraviolet radiation emitted by a fire. Flame detectors are particularly effective in environments where rapid detection of fire is critical, as they can respond quickly to the distinct energy produced by flames.

In contrast, ionization detectors are more sensitive to smoke particles and are designed to identify the smoke produced in a fire rather than directly detecting flames. Smoke detectors generally rely on the presence of smoke to trigger an alarm and do not respond directly to heat or flames. Heat detectors, while they do detect changes in temperature, typically respond to gradual increases in temperature and not directly to the flames or their radiant energy.

Therefore, flame detectors are uniquely suited for environments where detecting flame movement or radiant energy is essential, making them the correct choice for this question.