Fishes survive in water mainly because they possess

Analyze the question: It asks what mainly allows fishes to survive in water. Survival in water depends on being able to obtain oxygen there, so we are looking for the feature that makes this possible.

Explain the concept: Fishes are aquatic vertebrates. Their key adaptations are:

• Streamlined body: reduces resistance so they swim easily (aids movement).
• Scales: protect the body and reduce friction (protection).
• Paired fins (pectoral and pelvic): help with balance, steering and stopping (stability and movement).
• Gills: the respiratory organs. They extract dissolved oxygen from water and release carbon dioxide (gaseous exchange).

Step-by-step solution:

1. Every living organism needs oxygen for respiration, and without it the organism dies.
2. In water, oxygen is available only in dissolved form. Fishes cannot use lungs like land animals.
3. Gills have thin, highly vascularised filaments with a large surface area. Water flows over them, and dissolved oxygen diffuses into the blood while carbon dioxide diffuses out.
4. Streamlining, scales and fins help with movement and protection, but a fish could not stay alive without gills. Gills are therefore the main feature enabling survival in water.

Answer: The correct option is D. gills.

Exam tip: When a question says "survive" or "breathe" in water, think gills. When it says "move" or "swim," think streamlined body and fins. Match the function to the organ: gills = respiration, fins = balance and movement, scales = protection.