a) air fuel ratio
b) amount of fuel in air fuel mixture
c) amount of air in air fuel mixture
d) engine speed

Answer: a
Explanation: Air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process. The horsepower of an engine is the power output of the engine, usually available at the crankshaft. So, if an engine is rated at 100 HP, it means that at the crankshaft of the engine, 100 horsepower of mechanical power is available when the engine is operating.