The correct answer is option 4 - Lactic acid.

During muscular activity, the muscles produce energy through a process called glycolysis. Glycolysis breaks down glucose molecules into pyruvate, which is then further metabolized to produce energy. However, when the oxygen supply to the muscles is limited, such as during intense exercise or physical exertion, the pyruvate is converted into lactic acid instead.

The buildup of lactic acid in the muscles can lead to fatigue and a burning sensation. This happens because lactic acid lowers the pH level of the muscles, causing them to become more acidic. This change in pH affects the muscles` ability to contract and generate force, resulting in fatigue and decreased performance.

It is important to note that lactic acid is not the cause of delayed-onset muscle soreness (DOMS), which often occurs 24-48 hours after exercise. DOMS is believed to be caused by other factors, such as microscopic damage to the muscle fibers and inflammation.

In summary, the release of lactic acid is responsible for causing fatigue after muscular activity in human beings, as it affects the muscles` ability to contract and generate force.