The vessel in which the cracking reaction occurs or is completed, and in which the...

This definition is rooted in petroleum refining terminology used in the context of fluid catalytic cracking (FCC) units:

“Reactor: The vessel in which the cracking reaction occurs or is completed and in which the product gases are separated from coke and/or catalyst particulate.”

— API Std 936, Definitions, Section 3​

Option B is the correct term describing this component. A stripper removes volatiles, a regenerator burns off coke from spent catalyst, and a standpipe is used for catalyst transport—not for the cracking reaction itself.

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