Fossil fuels have been used for years as a source
of energy. even though there are many negative
issues associated with the use of fossil fuels, they
continue to be used to a great extent. this is
most likely because
(1) they have been commercially available as an
energy source
(2) there are alternatives to these types of fuels
(3) they have had a positive effect on global
temperatures
(4) fossil fuels can be burned to produce large
quantities of carbon dioxide

The best answer is (1)

Fossil fuels are commercially available as an energy source.

They are very easy to find. More and more extractions of fossil fuels is going on everyday. With advancement in science and  technology, the refineries and extraction procedures have improved a lot. This makes their availability even better.

Fossil fuels are also easily combustible. Most combustion engines need to be powered with little amount of fuel to produce large amount of energy. Very large amounts of electricity can be  generated  from fossil fuels.

humans are now responsible for causing changes in the environment that hurt animals and plant species. we take up more space on earth for our homes and cities. we pollute habitats. human activity often changes or destroys the habitats that plants and animals need to survive.

it creates acid rain