Discussion
The following is the most important requirement for laser :
Answer: B
The keyed requirement is stimulated emission.
The word laser stands for Light Amplification by Stimulated Emission of Radiation. An incoming photon causes an excited atom to emit a second photon of the same frequency, phase, direction and polarisation; this is what makes laser light coherent and allows amplification.
Pumping (optical or electrical) and the optical feedback of the resonator are needed to create population inversion and sustain oscillation, but they serve the stimulated emission process. Optical pumping is only one way of pumping (diode lasers are pumped by current), and current modulation is a way of encoding data, not a requirement for lasing.
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