Which correctly describes what can be learned from Hall effect measurements?

Answer: B. The sign of the Hall coefficient gives the type of majority carrier, and its magnitude gives the carrier concentration

When a magnetic field is applied perpendicular to a current-carrying sample, the Lorentz force pushes the carriers to one side, producing a Hall voltage in the direction perpendicular to both the current and the field. With a single type of carrier, the Hall coefficient is R_H = 1/(ne) for holes and R_H = −1/(ne) for electrons, so the sign gives the type of majority carrier and the magnitude gives the carrier concentration n. Combined with the resistivity, the mobility μ = |R_H|/ρ can also be found. The Hall voltage is proportional to both the current and the magnetic field. The band gap is obtained from the temperature dependence of the carrier concentration or from optical absorption. Hall elements in magnetic sensors exploit this effect.