Hint: The infrared radiations are emitted by most of the objects as thermal radiations. So the instrument which can detect the temperature of an object can be used in the detection of the infrared waves.
Complete answer: The spectrometer is a device which is used in the measurement of the physical characteristic of an electromagnetic radiation for a given spectral range. So, it cannot detect a particular range of the wavelengths of the EM spectrum separately. Therefore a spectrometer cannot be used to detect the infrared radiations. Hence, the option A is incorrect. A nanometer is a unit rather than being a device. There is no device which exists by the name of a nanometer. Hence, the option C is also incorrect. A photometer is a device which compares the radiations emitted by two sources. It is used for the measurement of the luminous intensity or the brightness by this comparison. It works in the region from the ultraviolet to the infrared of the EM spectrum. It also includes some of the visible spectrum. As it can work in a mixture of regions of the EM spectrum, so it cannot separately detect the infrared radiation. Hence, the option D is also incorrect. But the pyrometer, which is used as a remote sensing thermometer for the measurement of the temperature of the different surfaces, is found to be sensitive to the wavelengths corresponding to the infrared region of the EM spectrum. Therefore it can be used in the detection of the infrared radiations. Hence, the correct answer is option (B).
Note: The pyrometers are very much used in the metallurgical processes for the continuous measurement of the temperature of the metallurgical furnaces. It is also used in a hot air balloon for preventing the overheating of the top of the balloon. Also in the sun bath furnaces, the pyrometers are used for maintaining the precision in the heat treatment processes occurring over there.