Carburetors are devices used in internal combustion engines to mix air and fuel in the correct proportions for combustion. The mixture supplied by the carburetor during different engine operating conditions determines the efficiency and performance of the engine.

The options provided in the question are related to the air-fuel mixture supplied by the carburetor during different operating conditions - starting, idling, and low-speed operation. Let's discuss each option in detail:

a) Lean mixture:
A lean mixture refers to a mixture where the air-to-fuel ratio is higher than the stoichiometric ratio. In a lean mixture, there is an excess of air compared to the amount of fuel. This type of mixture is used during high-speed operation or when the engine requires more power. A lean mixture helps in achieving better fuel economy and lower emissions.

b) Slightly leaner than stoichiometric mixture:
The stoichiometric mixture refers to the ideal air-fuel ratio required for complete combustion. It is the ratio at which all the fuel is burned with the available oxygen in the air. During normal operating conditions, the carburetor supplies a mixture slightly leaner than stoichiometric to improve fuel economy and reduce emissions.

c) Stoichiometric mixture:
The stoichiometric mixture, as mentioned earlier, is the ideal ratio of air to fuel required for complete combustion. It ensures that all the fuel is burned completely, resulting in efficient combustion and maximum power output. However, during starting, idling, and low-speed operation, the carburetor does not supply a stoichiometric mixture.

d) Rich mixture:
The correct answer to the question is option 'D' - rich mixture. A rich mixture refers to a mixture where the air-to-fuel ratio is lower than the stoichiometric ratio. In a rich mixture, there is an excess of fuel compared to the available oxygen. During starting, idling, and low-speed operation, the carburetor supplies a rich mixture to ensure easy starting, smooth idling, and better throttle response. A rich mixture provides additional fuel for combustion under these conditions, compensating for the reduced air intake.

In summary, the carburetor supplies a rich mixture during starting, idling, and low-speed operation. This ensures easy starting, smooth idling, and better throttle response by compensating for the reduced air intake during these operating conditions.

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