Under the Central Pollution Control Board (CPCB) General Standards for Discharge of Environmental Pollutants (Schedule VI of Environment Protection Rules), the maximum permissible Biochemical Oxygen Demand ($BOD_3$ at $27^\circ\text{C}$) for industrial and food processing effluents discharged into public sewers is $350\text{ mg/L}$ (whereas for discharge into inland surface water it is $30\text{ mg/L}$, and for land for irrigation it is $100\text{ mg/L}$).

Option analysis:

• $50$: Below the sewer standard (inland surface water standard is $30\text{ mg/L}$).
• $150$: Not the regulatory limit for public sewer discharge.
• $350$: Correct, the maximum permissible BOD limit for discharge into public sewers.
• $600$: Exceeds permissible statutory effluent discharge limits.

In a fire tube boiler, hot combustion gases pass through tubes surrounded by water inside the boiler shell. The Locomotive boiler (along with Cochran, Lancashire, and Cornish boilers) is a classic fire tube boiler. In contrast, Babcock and Wilcox, Benson, and Stirling boilers are water tube boilers where water circulates inside the tubes while hot flue gases pass externally.

Option analysis:

• Babcock and Wilcox boiler: High-pressure water tube boiler.
• Locomotive boiler: Correct, horizontal multi-tubular externally fired fire tube boiler.
• Benson boiler: Supercritical once-through water tube boiler.
• Stirling boiler: Bent-tube water tube boiler.

Level measurement techniques are divided into direct (contact) and indirect (non-contact/inferred) methods. Ultrasonic (sonar) sensors measure the transit time of high-frequency sound waves reflected from the liquid interface without physical contact with the fluid, making it a premier indirect/non-contact measurement method.

Option analysis:

• Conductive probes: Direct contact method relying on electrical conduction across the fluid.
• Sonar or ultrasonic: Correct, non-contact indirect level measurement using acoustic echo transit time.
• Displacer: Direct contact buoyancy-based measurement technique.
• Sight glass: Direct visual contact method using a transparent tube connected to the vessel.