The correct answer is option 'D' - more than 900.

Explanation:
Amino acids are organic compounds that are the building blocks of proteins. They consist of an amino group (-NH2), a carboxyl group (-COOH), and a side chain that varies for each amino acid. In biological systems, there are more than 900 known amino acids.

1. The 20 Standard Amino Acids:
The 20 standard amino acids are the most commonly found amino acids in proteins. They are:

- Alanine (Ala)
- Arginine (Arg)
- Asparagine (Asn)
- Aspartic acid (Asp)
- Cysteine (Cys)
- Glutamine (Gln)
- Glutamic acid (Glu)
- Glycine (Gly)
- Histidine (His)
- Isoleucine (Ile)
- Leucine (Leu)
- Lysine (Lys)
- Methionine (Met)
- Phenylalanine (Phe)
- Proline (Pro)
- Serine (Ser)
- Threonine (Thr)
- Tryptophan (Trp)
- Tyrosine (Tyr)
- Valine (Val)

These 20 amino acids are essential for the synthesis of proteins in living organisms.

2. Non-Standard Amino Acids:
Apart from the 20 standard amino acids, there are also non-standard amino acids that are found in biological systems. These non-standard amino acids may have modified side chains or additional functional groups. Some examples include:

- Selenocysteine: Contains selenium in place of sulfur in the side chain.
- Pyrrolysine: Contains a pyrroline ring in the side chain.
- Norleucine: Similar to leucine but lacks one methyl group in the side chain.

3. Amino Acids in Non-Protein Functions:
Amino acids are not only involved in protein synthesis but also play important roles in various non-protein functions. For example:

- Neurotransmitters: Amino acids like glutamate, glycine, and gamma-aminobutyric acid (GABA) act as neurotransmitters in the nervous system.
- Precursors for Molecules: Amino acids are precursors for the synthesis of molecules like heme, purines, pyrimidines, and creatine.

Overall, the total number of known amino acids in biological systems exceeds 900, including the 20 standard amino acids and various non-standard amino acids.