The correct answer is option 1: doubled.

When a heater coil is cut into two equal parts, each part will have half of the original length. The heat generated in a heater coil is directly proportional to its length. Therefore, when only one part of the coil is used in the heater, the length of the coil is halved, resulting in a reduction in the heat generated.

However, since the length of the coil is halved, the resistance of the coil is also halved. According to Ohm`s law (V = IR), when the resistance decreases, the current passing through the coil increases (assuming the voltage remains constant). The heat generated in a heater coil is also directly proportional to the square of the current passing through it.

Therefore, the decrease in resistance is compensated by the increase in current, resulting in the heat generated being doubled compared to when the full length of the coil was used.

It is important to note that if the heater uses both parts of the cut coil, then the heat generated would be the same as when the full length of the coil was used.