Concept:
Flame photometry is an analytical technique used mainly to measure elements like sodium, potassium, calcium and lithium. The question is about the basic principle — what exactly the instrument measures.
Step 1: In flame photometry, the sample solution is sprayed into a flame. The heat of the flame excites the metal atoms, pushing their electrons to higher energy levels.
Step 2: These excited electrons are unstable and quickly fall back to their lower energy levels. As they do, they release the extra energy as light of a characteristic colour/wavelength. This is emission of radiation, and the brightness of the emitted light is proportional to the amount of the element present.
Step 3: So flame photometry is based on measuring emitted radiation. This is different from atomic absorption spectroscopy, which measures absorbed radiation instead. Scattering and reflection are not the basis of this method.
Answer: Option (3) — Emitted radiation.