Mechanisms of generalized and specialized bacterial transduction:
Statement (A) is correct: In abortive transduction (or specialized transduction by defective prophages like $\lambda dgal$), the transferred donor DNA fragment persists as a non-replicating extrachromosomal element, creating a partial diploid (merodiploid) cell containing two copies of that specific gene locus.
Statement (B) is incorrect: Complete transduction results in stable homologous recombination into the recipient chromosome with replacement of the recipient allele, whereas the persistent partially diploid condition arises specifically from ABORTIVE or defective transduction, NOT complete transduction.

Chemical catalytic mechanism of DNA ligase:

• DNA ligase seals single-strand nicks in double-stranded DNA by catalyzing the formation of a covalent phosphodiester bond between an adjacent 3'-hydroxyl group and a 5'-phosphate group.
• The reaction requires either ATP (in bacteriophage T4 and eukaryotes) or $\text{NAD}^+$ (in E. coli) as a cofactor to activate the 5'-phosphate.

Mechanism of Ethidium Bromide (EtBr) fluorescence:

• Ethidium Bromide is a planar polycyclic aromatic dye that intercalates between stacked nitrogenous base pairs of double-stranded DNA.
• Upon exposure to UV radiation (around 300 nm transilluminator), intercalated EtBr absorbs UV light and re-emits intense orange-red visible fluorescence (around 590 nm), with a 20- to 25-fold fluorescence enhancement compared to free dye in solution.
• Bromophenol blue is a tracking dye, silver nitrate is an all-around non-fluorescent protein/DNA stain, and X-Gal is a chromogenic substrate for beta-galactosidase.