Questions: How many amino acids are considered to be essential?

The answer is the third one (or C): 9.

Explanation for each option:

• 8: This is incorrect. While it is close, it does not account for all the essential amino acids.
• 15: This is incorrect. There are not 15 essential amino acids.
• 9: This is correct. There are 9 essential amino acids that humans must obtain from their diet because the body cannot synthesize them.
• 20: This is incorrect. There are 20 standard amino acids, but not all of them are essential.

Summary: There are 9 amino acids that are considered essential for humans. These must be obtained through the diet as the body cannot produce them on its own.