The correct answer is: 5 days.
Biological oxygen demand (BOD) is a measure of the amount of oxygen that is used by microorganisms to break down organic matter in water. The higher the BOD, the more organic matter is present in the water and the greater the demand for oxygen.
The standard BOD test is conducted over a period of five days. During this time, the microorganisms in the water consume oxygen, and the amount of oxygen consumed is measured. The BOD5 value is reported as milligrams of oxygen per liter (mg/L) of water.
A high BOD value indicates that the water is polluted with organic matter. This can cause problems for aquatic life, as the depletion of oxygen can lead to fish kills. BOD is also an important indicator of the effectiveness of wastewater treatment plants.
The BOD test is a simple and inexpensive way to measure the quality of water. It is widely used by environmental scientists, engineers, and regulators.
The other options are incorrect because they are not the standard time period for the BOD test.