The correct answer is A. Higher temperature.
A higher temperature can retard the self purification of a stream because it increases the rate of chemical reactions, including those that break down organic matter. This can lead to a decrease in dissolved oxygen levels, which can harm aquatic life.
Sunlight is necessary for photosynthesis, which produces oxygen. Therefore, sunlight can help to improve the self purification of a stream.
Satisfying oxygen demand means that there is enough oxygen in the water to support aquatic life. This can help to improve the self purification of a stream by preventing the death of aquatic life, which can release harmful pollutants into the water.
Therefore, the correct answer is A. Higher temperature.