Operational characteristics of positive displacement (PD) pumps:

• Statement (A) is true: PD pumps operate efficiently against high discharge pressures with moderate/low flow rates.
• Statement (B) is true: flow rate in PD pumps is directly proportional to rotational/stroking speed ($Q \propto N$).
• Statement (C) is false: because displacement per stroke/revolution is fixed and independent of backpressure, PD pumps are universally used as precision metering and dosing pumps.
• Statement (D) is false: because a PD pump will continue delivering flow regardless of downstream line blockages, an overpressure relief valve is strictly mandatory to prevent pump and pipe burst.
  Since (C) and (D) are not true, Option 4 is the correct choice.

Theories of dimensional analysis in engineering:

• Casson's model governs plastic fluid flow.
• Davis's theorem relates to electrical circuit theory.
• Buckingham's $\Pi$ (Pi) Theorem is the foundational theorem of dimensional analysis in fluid mechanics and heat transfer, stating that if an equation involving $n$ physical variables has $m$ fundamental dimensions, it can be reformulated into $(n - m)$ independent dimensionless $\Pi$-groups (e.g., Reynolds, Nusselt, Prandtl numbers).
• Maxwell's theorem relates to reciprocal deflections in structural mechanics.

Nomenclature of halocarbon refrigerants (ASHRAE numbering system):

• Monochloro-monofluoromethane is $CH_2ClF$ (R-31).
• Freon 22 (R-22, Chlorodifluoromethane) has the chemical formula $CHClF_2$, representing monochloro-difluoro methane.
• Dichlorodifluoromethane ($CCl_2F_2$) is Freon 12 (R-12).
• Dichloromonofluoromethane ($CHCl_2F$) is Freon 21 (R-21).