There's two forms of latent heat, there's latent heat of fusion which is melting, and there's latent heat of vaporization which is boiling. They describe the direction of energy flow when changing from one thing to the next like solid to liquid, or liquid to gas.
If you have a lump of solid at its melting point ... like ice at 32°F ... you have to put a certain amount of heat into it just to change it to water at 32°F. That amount of heat, that's used just to change a solid lump into liquid without changing its temperature, is called the heat of fusion for that substance.
The number is different for every substance.
For water, it takes 336 joules of heat to melt 1 gram of ice into 1 gram of water, all at 32°F (0°C). That's an enormous latent heat of fusion ... more than almost any other known substance. That's why ice is such a good choice when you need something to put in your drink to cool it down. Ice absorbs a huge amount of heat before it melts and the drink gets watered down.
As given, the student has three balloons and rubs two of them on a piece of wool. The rubbing of balloon on wool is the independent variable as it was done on two and not on the third as control.
