Brain derives energy from ketogenesis, if glucose is

The correct option is "less".

Ketogenesis is a metabolic pathway through which the liver produces ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) from fatty acids.
Under normal physiological conditions, the brain relies almost exclusively on glucose as its primary source of energy, consuming about 120 grams of glucose daily.

However, when the availability of glucose is less (such as during prolonged fasting, starvation, or a very low-carbohydrate ketogenic diet), blood glucose and insulin levels drop.
This hormonal shift triggers the mobilization of fatty acids from adipose tissue to the liver, where they undergo beta-oxidation and are converted into ketone bodies via ketogenesis.

These ketone bodies are released into the bloodstream and can cross the blood-brain barrier.
The brain adapts to use these ketone bodies as an alternative, highly efficient fuel source to meet its energy demands when glucose levels are low, thereby sparing glucose for other critical functions.