**Answer:**

The correct answer is option 'A', which is three.

**Explanation:**

To understand why the answer is three, let's first understand the structure and function of messenger RNA (mRNA). mRNA is a type of RNA molecule that carries genetic information from DNA to the ribosomes, where it is used as a template to synthesize proteins. Proteins are made up of amino acids, and the sequence of nucleotides in mRNA determines the sequence of amino acids in a protein.

**Codons:**
The sequence of nucleotides in mRNA is read in groups of three, known as codons. Each codon codes for a specific amino acid. There are a total of 64 possible codons, and each codon codes for one of the 20 different amino acids found in proteins. This means that there are multiple codons that can code for the same amino acid, which provides redundancy and flexibility in the genetic code.

**Example:**
For example, the codon "AUG" codes for the amino acid methionine, which serves as the start codon for protein synthesis. Other codons, such as "UAA", "UAG", and "UGA", are stop codons that signal the end of protein synthesis.

**Conclusion:**
In conclusion, a sequence of three nucleotides in mRNA makes a codon for an amino acid. This is because the genetic code is read in groups of three nucleotides, and each three-nucleotide sequence codes for a specific amino acid. Therefore, the correct answer is option 'A' - three.