The scientist is studying the He quarks. The answer is Letter D.
Particle accelerators are machines that accelerates rudimentary particles such as electrons or protons to higher energy levels. The energy of a beam of particles are speed up and incremented through the use of electric fields and are steered and focused through magnetic fields. The said beam particles travels inside a metal beam pipe in a vacuum. This is crucial for it helps in maintaining and air and dust free environment allowing the beam particles to travel unobstructed. Consequently, electromagnets steer and focus the beam particles as it travel through a vacuum tube. Electric fields then spaced around the accelerator switch from a positive state to a negative state at a given frequency producing radiowaves that accelerate particles in groups. Particles can be directed at a fixed target, such as a thin piece of metal foil or two beams of particles that can be collided. Finally, the particle detectors record and reveal the particles and radiation that are being produced by the collision between beam of particles and the target.
Base on the question, since the scientist wants to study the particles inside the proton, it can be deducted that a sub-component of proton is being studied which are technically called as QUARKS and gluons.