Which of the following effects can result from poor combustion?

Poor combustion in an engine typically means that the fuel is not being burned completely or efficiently. This can lead to higher emissions due to the incomplete combustion of hydrocarbons, resulting in the release of harmful pollutants such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons into the atmosphere. These emissions not only contribute to environmental pollution but can also result in failure to meet regulatory compliance standards.

The other aspects of engine operation, like efficiency and performance, are adversely affected by poor combustion, leading to decreased efficiency and performance levels, rather than improvements. Additionally, the costs associated with maintenance tend to rise when combustion is poor, as issues like fouled spark plugs, dirty fuel injectors, or clogged filters may require more frequent attention and repairs. Therefore, higher emissions is the correct outcome associated with poor combustion, making it the right choice in this context.