Let's analyze each statement:
a. The human body uses 20 amino acids to form all types of proteins - True. The human body utilizes 20 different amino acids to synthesize the various proteins necessary for bodily functions.
b. Amino acids do not need to be assembled in a particular order - False. Amino acids must be assembled in a specific order dictated by the genetic code in DNA to create proteins with the correct structure and function.
c. tRNA is attached to specific proteins in the cell's cytoplasm - False. tRNA (transfer RNA) is not attached to proteins; rather, it carries specific amino acids to the ribosome during protein synthesis, where it pairs with the corresponding codons on the mRNA.
d. Amino acids from a polypeptide chain via carbon bonds - False. Amino acids are linked together to form a polypeptide chain via peptide bonds, which are a type of covalent bond formed between the amino group of one amino acid and the carboxyl group of another.
Based on this analysis, the true statement is a. the human body uses 20 amino acids to form all types of proteins.