Which of the following is a method by which recreational waters may be contaminated?

Recreational waters, such as lakes, rivers, and swimming pools, can be contaminated through various means, illustrating the multifaceted nature of water quality issues.

Infected people can contaminate recreational waters primarily through the shedding of pathogenic microorganisms in feces, which can occur in settings like public swimming pools or beaches. When infected individuals engage in recreational activities, these pathogens can enter the water, posing health risks to others.

Surface runoff plays a significant role in water contamination as well. During rainfall, water can wash contaminants from the land, including agricultural runoff containing fertilizers, pesticides, and animal waste, into nearby bodies of water. This runoff can lead to nutrient loading, increased pathogens, and degradation of water quality.

While normal water flora, which includes naturally occurring microorganisms, typically contribute to the ecosystem's balance, certain situations can lead to imbalances, especially if external nutrients or pollutants cause harmful algal blooms. These blooms can produce toxins, further compromising the safety of recreational waters.

Therefore, all of these factors—infected individuals, surface runoff, and the presence of flora under altered conditions—contribute to the contamination of recreational waters, making the collective response the most appropriate choice.