The correct answer is: C. Cyanometer.
A cyanometer is an instrument used to measure the blueness of the sky or ocean. It is a type of spectrometer that measures the intensity of light at different wavelengths. The intensity of light at blue wavelengths is used to calculate the blueness of the sky or ocean.
A bathymeter is an instrument used to measure the depth of water. It is typically used to measure the depth of oceans, lakes, and rivers.
A ceraunograph is an instrument used to measure the frequency of lightning strikes. It is typically used to measure the frequency of lightning strikes in a particular area.
A barometer is an instrument used to measure atmospheric pressure. It is typically used to measure the pressure of the atmosphere at a particular location.