The correct option for the conversion of heat into electrical energy is option 4, Thermocouple. A thermocouple is a device that consists of two different metal wires connected at two junctions. When one junction is exposed to heat, it creates a temperature difference between the junctions. This temperature difference generates a voltage, known as the Seebeck effect, which can be measured and converted into electrical energy.
Option 1, Ammeter, is incorrect as it is a device used to measure the flow of electric current, not convert heat into electrical energy.
Option 2, Hydrometer, is also incorrect as it is a device used to measure the specific gravity or density of a liquid, not convert heat into electrical energy.
Option 3, Voltmeter, is incorrect as well, as it is a device used to measure the electrical potential difference between two points, not convert heat into electrical energy.
Therefore, the correct option is option 4, Thermocouple, as it is specifically designed to convert heat into electrical energy.