Which of the following is a well-known poisonous chemical found in apple seeds?

The correct answer is cyanide, which is indeed a well-known poisonous chemical found in apple seeds. Apple seeds contain amygdalin, a compound that can release cyanide when metabolized in the body. Although the amount of cyanide present in a single apple seed is quite small and generally not harmful in the quantities typically consumed, ingesting a large number of seeds could potentially lead to cyanide poisoning. This makes it important to understand the potential dangers associated with consuming apple seeds, despite apples being a healthy fruit overall.

The other options listed, while they may also be considered toxic substances, are not related to apple seeds. Arsenic is a well-known poison often associated with contaminated water or certain pesticides, but it is not naturally found in apple seeds. Formaldehyde is a chemical commonly used for preserving biological specimens and in various industrial applications, but it doesn't have a connection to apple seeds. Lead is a heavy metal that can be toxic, particularly in cases of contamination, but it is not found in apple seeds either.