An engine's compression ratio compares

Correct answer to question 57 (Auto & Shop Information): cylinder volume at the bottom of the stroke with volume at the top

Method

It is how far the mixture is squeezed: a ratio of 10 to 1 means the space at the top is a tenth of the space at the bottom. Higher ratios extract more work but demand higher octane fuel, and diesels run far higher ratios than petrol engines. The second choice describes the air-fuel ratio, a different figure.