A five-carbon ketose is known as: 

CONCEPT:

Ketose and Pentulose

• A ketose is a monosaccharide (simple sugar) that contains a ketone group in its structure.
• The number of carbon atoms in the sugar determines its classification. For example:
  • Three-carbon ketose: Triose
  • Four-carbon ketose: Tetrose
  • Five-carbon ketose: Pentose
  • Six-carbon ketose: Hexose
• A five-carbon ketose is specifically known as a pentulose because "pent-" refers to five carbon atoms and "-ulose" indicates it is a ketose.

EXPLANATION:

• In the given question:
  • The sugar is a ketose, meaning it contains a ketone group.
  • Since the sugar has five carbon atoms, it falls under the category of pentose.
  • Combining both features (ketone group and five carbon atoms), it is classified as a pentulose.
    
• Other options are incorrect:
  • Option 2 (Ketohexose): Refers to a six-carbon ketose, not five-carbon.
    
  • Option 3 (Arabinose): Arabinose is an aldose, not a ketose.
  • 
  • Option 4 (Ribose): Ribose is also an aldose, not a ketose.
    

Therefore, the correct answer is Option 1: Pentulose.