Choose the correct statement about thermal cracking of petroleum hydrocarbons with respect to operating conditions and product distribution.

Introduction:
Thermal cracking breaks larger hydrocarbon molecules into smaller ones by heat (and often steam dilution). Pressure, temperature, and residence time govern severity and the distribution of lighter products and olefins.

Given Data / Assumptions:

• Process: non-catalytic thermal cracking.
• Variables: temperature, pressure, residence time.
• Focus: trend of lighter hydrocarbon yield.

Concept / Approach:
Lower pressure generally shifts equilibria and kinetics to favour molecule scission and suppress recombination, increasing lighter products and olefinicity at given temperature severity.

Step-by-Step Solution:
1) Recognize cracking severity depends on T, P, and time.2) Lower pressure limits secondary recombination, enhancing light ends.3) Therefore, low pressure tends to increase lighter hydrocarbon yield.

Verification / Alternative check:
Steam cracking (extremely low partial pressure via steam dilution) is an extreme case showing high light olefin yields, illustrating the low-pressure effect.

Why Other Options Are Wrong:
Temperature insensitivity: False; severity is highly temperature-dependent.Residence time: Increasing time generally increases severity, not decreases it.Lower octane with greater depth: Deeper cracking usually increases olefins/aromatics that enhance octane; the statement is misleading.High pressure maximizes olefins: Opposite trend; high pressure favours recombination.

Common Pitfalls:
Assuming pressure only affects equipment design; in cracking, pressure materially alters radical recombination and product distribution.

Final Answer:
At low pressure, the yield of lighter hydrocarbons increases.