Which prefix pertains to water or hydrogen?

• A. Eti/o
• B. Calc/i
• C. Leuk/o
• D. Hydr/o

Answer: (D)

The meaning being tested is about a combining form that denotes water or hydrogen. Hydr/o conveys water (and by extension hydrogen in many terms), making it the appropriate prefix here. That form is used in words like hydrotherapy or hydrolysis, where the-water idea is central.

The other options map to different ideas: eti/o means cause or etiology, not water; calc/i relates to calcium or calcification; leuk/o means white (as in white blood cells). So they don’t fit the concept of water or hydrogen.