The correct answer is: C. Atomic clocks.
Atomic clocks are the most accurate timekeepers ever devised. They are so precise that they can measure time to within a billionth of a second. This makes them essential for a variety of applications, including navigation, telecommunications, and scientific research.
Atomic clocks work by measuring the frequency of light emitted by atoms. The frequency of light is determined by the energy levels of the atoms, which are in turn determined by the mass of the atoms and the strength of the electromagnetic force. By measuring the frequency of light with great precision, atomic clocks can determine the time with great accuracy.
Pulsars are rapidly rotating neutron stars that emit a beam of radiation. The rotation of the pulsar causes the beam to sweep across the Earth, and we see a series of pulses of radiation. The frequency of the pulses is determined by the rotation rate of the pulsar, which can be measured with great precision. This makes pulsars useful for timekeeping, but they are not as accurate as atomic clocks.
Quartz clocks are based on the piezoelectric effect, which is the ability of certain materials to generate an electric charge when they are deformed. In a quartz clock, a quartz crystal is deformed by a small electric current, and the resulting electric charge is used to control the frequency of a oscillator. The oscillator is then used to control the timekeeping mechanism of the clock. Quartz clocks are very accurate, but they are not as accurate as atomic clocks.
White dwarfs are the remnants of stars that have exhausted their nuclear fuel. They are very dense objects, with a mass that is typically about the same as the Sun but a radius that is only about the size of the Earth. The surface of a white dwarf is very hot, and it emits a great deal of radiation. However, the radiation from a white dwarf is not very stable, and it can vary in intensity over time. This makes white dwarfs unsuitable for timekeeping.