How many standard amino acids are commonly found in proteins?

• A. $24$
• B. $22$
• C. $20$
• D. $18$

Answer: (C)

Verified step by step guidance

1. Understand that amino acids are the building blocks of proteins, and there are standard amino acids that are commonly found in proteins.
2. Recognize that the term 'standard amino acids' refers to those amino acids that are encoded by the universal genetic code.
3. Recall that there are 20 standard amino acids that are commonly found in proteins, which are encoded by the genetic code in most organisms.
4. Note that while there are other amino acids that can be found in proteins, such as selenocysteine and pyrrolysine, these are not considered part of the standard set.
5. Conclude that the number of standard amino acids commonly found in proteins is 20, based on the universal genetic code.