Comprehensive and Detailed Explanation From Nokia Optical Networking Fundamentals:

Coherent transmission represents a massive leap in optical technology, moving beyond simple 'on-off keying' (Intensity Modulation) to more complex modulation formats like QPSK or 16-QAM. A fundamental requirement of a coherent receiver is the ability to recover and track the carrier phase information of the incoming signal. This is achieved by using a Local Oscillator (LO) laser at the receiver that interferes with the incoming signal, allowing the receiver to extract phase and polarization data.

Unlike legacy 10G direct-detection systems, coherent systems (like Nokia's PSE-V engine) perform Digital Signal Processing (DSP) to electronically compensate for impairments. This makes Option D false, as physical Dispersion Compensation Modules (DCMs) are actually detrimental and usually removed in coherent networks. Option B is incorrect as coherent transmission is designed for Single-Mode Fiber (SMF). Option C refers to Flex-grid technology; while coherent signals often use Flex-grid, the defining characteristic of coherent technology is the phase-sensitive detection at the receiver.