The instrument used for detecting the presence of electric current in a circuit is a Galvanometer. A Galvanometer is a device that is designed to measure and detect electric current. It works based on the principle of electromagnetic induction, which means that when an electric current flows through a wire, it creates a magnetic field around it. The Galvanometer utilizes this principle by using a coil of wire suspended in a magnetic field. When electric current passes through the coil, it experiences a force due to the magnetic field, causing it to rotate. This rotation is then converted into a reading on a scale, indicating the presence and magnitude of the electric current.

Option 1, the refractometer, is a device used to measure the refractive index of a substance, not for detecting electric current.

Option 3, the viscometer, is a device used to measure the viscosity or thickness of a fluid, not for detecting electric current.

Option 4, the diffractometer, is a device used to analyze the diffraction patterns of materials, not for detecting electric current.

Therefore, the correct answer is option 2, Galvanometer.