Universality of genetic code and mitochondrial exceptions:

• Assertion (A) is true: The genetic code is nearly universal, but not completely absolute, because deviations occur in mitochondria and certain protozoans.
• Reason (R) is true: In mammalian and vertebrate mitochondria, the codon UGA, which serves as a stop signal in the standard code, specifies Tryptophan (Trp).
• Reason (R) directly explains Assertion (A) by providing empirical proof of codon reassignment that breaks strict universality.
• Therefore, both (A) and (R) are true and (R) is the correct explanation of (A).

Structural mimicry of puromycin:

• Puromycin does not mimic alanyl-tRNA.
• Puromycin lacks imidazole functionality and does not resemble histidyl-tRNA.
• Puromycin is a structural analog of the $3'$ terminal adenosine of tyrosyl-tRNA (tyrosinyl-tRNA); it enters the ribosomal A site and accepts the nascent polypeptide, forming peptidyl-puromycin that causes premature termination of translation.
• Prolyl-tRNA has an imino acid structure and does not resemble puromycin.

Evaluation of post-translational modifications (PTMs):

• Statement (A) is true: Proteolytic cleavage of polyprotein precursors into individual active proteins (e.g., viral polyproteins, insulin precursor) is a major PTM.
• Statement (B) is true: Post-translational hydroxylation of proline and lysine residues by prolyl and lysyl hydroxylases is essential for collagen stability.
• Statement (C) is false: In prokaryotes, formylation of methionine occurs before translation begins on the loaded initiator tRNA ($f\text{Met-tRNA}^{\text{fMet}}$); hence it is a pre-translational event.
• Statement (D) is true: Disulfide bridge formation between cysteine residues occurs post-translationally in the endoplasmic reticulum.