Rating Scale for Grading Gasoline
Puzzle 7 of Energy Sources Hard Pack
Question is: Rating Scale for Grading Gasoline
and the answer is: OCTANE
Word Meaning:
OCTANE — (organic chemistry) Any of the eighteen isomeric aliphatic hydrocarbons (C₈H₁₈) found in petroleum, especially an iso-octane 2,2,4 trimethyl-pentane; they are used as fuels and solvents.
Definitions from Wiktionary.