The correct answer is option 'D' - Charging of tRNA.

Charging of tRNA is the process in which amino acids are activated in the presence of ATP and then attached to their cognate tRNA molecules. This process is also referred to as aminoacylation or aminoacetylation of tRNA.

The process of charging of tRNA involves several steps, which can be summarized as follows:

1. Activation of amino acids:
- The process begins with the activation of amino acids in the presence of ATP. Each amino acid is specifically activated by a specific aminoacyl-tRNA synthetase enzyme.
- The ATP molecule is hydrolyzed to AMP (adenosine monophosphate) and pyrophosphate (PPi), releasing energy in the process.
- The activated amino acid is formed when the carboxyl group of the amino acid reacts with the AMP moiety, resulting in the formation of an aminoacyl-AMP intermediate.

2. Transfer of amino acid to tRNA:
- The activated amino acid is then transferred to its cognate tRNA molecule, which possesses a specific anticodon sequence that corresponds to the codon on the mRNA during translation.
- The transfer is catalyzed by the same aminoacyl-tRNA synthetase enzyme that activated the amino acid.
- The enzyme recognizes both the specific amino acid and the specific tRNA molecule, ensuring accurate pairing between the amino acid and the tRNA.

3. Formation of aminoacyl-tRNA:
- The amino acid is transferred from the aminoacyl-AMP intermediate to the 3' end of the tRNA molecule, forming an aminoacyl-tRNA complex.
- This complex is now ready to be used in protein synthesis during translation.

The charging of tRNA is an essential step in protein synthesis as it ensures that the correct amino acid is attached to the corresponding tRNA molecule. This process is highly specific and accurate, thanks to the specificity of the aminoacyl-tRNA synthetase enzymes.

Overall, the activation of amino acids in the presence of ATP and their subsequent attachment to their cognate tRNA molecules is known as the charging of tRNA. This process plays a crucial role in the accurate translation of the genetic code into functional proteins.