Fusion power — Commercial power generation from a thermonuclear fusion reactor is not yet realized mainly because:

Introduction:
Fusion promises abundant, low-carbon energy, but commercial plants remain elusive. The central challenge is maintaining a stable, high-temperature plasma long enough (adequate confinement time and density) to achieve net energy gain.

Given Data / Assumptions:

• Fusion fuels (D and T) are available via seawater and breeding (T from lithium).
• Reactor concepts include magnetic confinement (tokamak, stellarator) and inertial confinement.
• Question asks for the main reason commercialization is not yet achieved.

Concept / Approach:
Net-positive fusion requires satisfying the Lawson criterion (sufficient product of density, temperature, and confinement time). Achieving and controlling these conditions without excessive losses or instabilities is the core obstacle.

Step-by-Step Solution:
Evaluate scarcity claim: Deuterium is abundant; tritium can be bred from lithium. Fuel scarcity is not primary.Evaluate initiation difficulty: Ignition is challenging, but the sustained control and confinement for net gain is the deeper issue.Evaluate fuel quantity: Required quantities are tiny; quantity is not the limiting factor.Therefore, the main barrier is control and confinement stability, making (a) the best answer.

Verification / Alternative check:
Research highlights include turbulence suppression, disruption avoidance, and materials able to withstand neutron fluxes, aligning with the control/confinement challenge.

Why Other Options Are Wrong:

• (b) Fuel scarcity is not the issue; D is abundant; T can be bred.
• (c) Initiation alone is not the key barrier; sustaining and controlling the plasma is.
• (d) Fuel quantity needed is very small, not prohibitively high.

Common Pitfalls:
Equating ignition with commercialization; ignoring engineering challenges in confinement, materials, and tritium breeding.

Final Answer:
it is difficult to control fusion reaction (confinement and stability challenges).