Amino acids for protein synthesis are first activated by:Group of answer choicesPhosphorylationAcetylationAdenylationMethylationUbiquitination

To determine the correct answer to the question about how amino acids are activated for protein synthesis, let's look at what each process entails:

1. Phosphorylation: This involves the addition of a phosphate group to a molecule, typically to change the activity of enzymes or to activate them. It is not directly involved in the activation of amino acids for proteins.

2. Acetylation: This is the addition of an acetyl group to a substrate. While it plays a role in various biological processes, it does not pertain specifically to the activation of amino acids for protein synthesis.

3. Adenylation: This is the addition of an adenosine monophosphate (AMP) or adenosine triphosphate (ATP) group, which is known to play a key role in activating amino acids.

4. Methylation: This is the addition of a methyl group to the molecule. It is important in gene regulation but is not the process through which amino acids are activated for synthesis.

5. Ubiquitination: This is the process of attaching ubiquitin proteins to a substrate, usually to target it for degradation. This process does not activate amino acids.

Conclusion

The correct choice here is Adenylation, as amino acids are activated for protein synthesis through the formation of aminoacyl-tRNA, which involves the adenylation of the amino acid.

Final Answer: Adenylation.