"Boiling water" means pumping enough heat into it to raise its temperature to the boiling point.
"Type of heat transfer" means how do you do that ... pump the heat into the water ?
Well, there are exactly three ways to transfer heat from one place to another: Conduction, convection, and radiation. There aren't any other ways. You can use ANY of these types of heat transfer IF you can construct a way to use it to heat water.
Conduction: Maybe you could heat a steel ball until it's glowing red-hot, then drop it into the water.
Convection: Maybe you could heat the bottom of the bottle or kettle the water is in, and let convection circulate all the water up and down until the whole thing starts boiling.
Radiation: Maybe you could shine a heat lamp or sunshine onto the water, and wait until the water absorbs enough of the radiated heat to start boiling.
The way it's MOST OFTEN done is to light a fire under the water container. When you do that, here are the types of heat transfer involved:
-- Heat transfers from the fire to the bottom of the container by CONDUCTION.
-- The heat passes through from the outside of the bottom of the container to the inside by CONDUCTION.
-- When the heat gets inside, it heats up the bottom of the water by CONDUCTION.
-- The water circulates up and down in the container. Hot water rises from the bottom to the top, and cold water sinks from the top to the bottom. That's CONVECTION. Eventually, ALL of the water in the container is hot enough to boil.
