What causes flaming combustion to occur?

select one:
a. a solid fuel mixes with an adequate amount of liquid fuel to be able to sustain an open burning flame.
b. a gaseous fuel mixes with an adequate amount of oxygen and heats to ignition temperature.
c. a solid fuel encounters an open flame and entrains enough air to ignite.
d. a gaseous fuel encounters an external heat source and experiences piloted ignition.