What causes beer to become skunky?

Beer becomes skunky primarily due to exposure to sunlight and fluorescent light, which can lead to a chemical reaction involving hops. Hops contain compounds called iso-alpha acids, and when they are exposed to UV light, they can break down and form 3-methylbut-2-ene-1-thiol (MBT), the compound responsible for the characteristic skunky aroma. This is why many breweries choose to package their beer in brown bottles or cans; these materials provide better protection against light exposure compared to clear or green bottles.

The other choices do contribute to potential beer quality issues, but they do not specifically lead to the skunky aroma. For example, exposure to oxygen can cause off-flavors and oxidation, high fermentation temperatures can result in unwanted esters or phenols, and improper storage conditions might lead to other spoilage but not necessarily skunkiness. Thus, the relationship between light exposure and the formation of skunky flavors is what makes this answer the most accurate.