Comprehensive and Detailed Explanation From Nokia Optical Networking Fundamentals:

While technologies like WSS (Wavelength Selective Switches) and coherent transmission (100G/200G/400G+) significantly improve the efficiency and reach of a network, the most direct way to increase the total transportable volume of services in a mesh topology is to upgrade link capacity or install new physical links. In Nokia optical planning, upgrading link capacity typically involves moving from a lower-rate system (like 10G) to a higher-rate system (like 100G or 400G) or increasing the number of available wavelengths by expanding from a 40-channel to an 80-channel or 96-channel C-band system.

Adding new links (new fiber spans) creates more degrees in the mesh, providing more paths for traffic and increasing the overall aggregate bandwidth of the network. Option A refers to flexibility (ROADM functionality) rather than raw capacity. Option B (PRC) relates to survivability and availability, not capacity expansion. While Option C (coherent transmission) is a powerful method for increasing capacity per wavelength, it is not the 'only' way, as adding more fiber (spatial multiplexing) or more channels (spectral density) are also primary methods for scaling a mesh network to handle more services.