The correct answer is option 3, which includes reasons 1, 2, and 3.

Explanation:

1. Mercury does not wet the inner sides of the thermometer: This means that mercury does not stick to the inner surface of the thermometer, allowing for more accurate measurement. If mercury were to wet the inner sides, it would form droplets instead of staying as a continuous column.

2. It can be seen easily in a thin capillary tube of the thermometer: This is another advantage of using mercury. Due to its silver-grey color and high density, mercury can be easily seen in the thin capillary tube of the thermometer, making it easier for the observer to read the temperature.

3. It is a good conductor of heat: Mercury is a good conductor of heat, which means that it quickly adjusts to the temperature of the object being measured. This allows for faster and more accurate temperature readings.

Option 4, which includes reasons 3 and 4, is incorrect because the non-toxicity of mercury is not a reason for using it in thermometers. In fact, mercury is a toxic substance and caution should be taken when handling it.