Pioneering scientists in plant resistance and genetic variation:

• J.E. Vanderplank (1963) formulated the concepts of vertical resistance (race-specific) and horizontal resistance (non-specific/general): (A)-(IV).
• Harold Flor (1942, 1956) proposed the Gene-for-Gene hypothesis working with flax rust (Melampsora lini): (B)-(III).
• N.I. Vavilov (1920) proposed the Law of Homologous Series in Variation (parallel variation): (C)-(I).
• R.H. Painter (1951) classified insect resistance mechanisms into non-preference (antixenosis), antibiosis, and tolerance: (D)-(II).

Thermal stability of double-stranded DNA:

• Melting temperature ($T_m$) increases linearly with increasing GC content: Assertion (A) is true.
• G-C base pair forms three hydrogen bonds ($G \equiv C$), whereas an A-T base pair forms only two ($A = T$), along with stronger $\pi-\pi$ base stacking interactions for GC pairs, requiring significantly more thermal energy to denature: Reason (R) is true and directly explains Assertion (A).

Ribozyme activity of ribosomal RNA:

• Ribosomal RNA (rRNA) constitutes the core catalytic center of the ribosome.
• Specifically, the large ribosomal subunit's $23\text{S}$ rRNA (prokaryotes) or $28\text{S}$ rRNA (eukaryotes) functions as a peptidyl transferase ribozyme, catalyzing peptide bond formation during translation.
• mRNA serves solely as a coding template; tRNA is an adaptor molecule; siRNA mediates RNA-induced silencing.