An association between living organisms in which one lives on and feeds at the expense of the other is known as

Step 1: Analyze the question The question describes a relationship where one organism lives on another and feeds at its expense. So one organism benefits and the other is harmed.

Step 2: Explain the concepts

• Commensalism: One organism benefits and the other is neither helped nor harmed. Example: an epiphyte (like an orchid) growing on a tree.
• Predation: One organism (the predator) kills and eats another (the prey). The predator does not live on the prey. Example: a lion hunting a zebra.
• Parasitism: One organism (the parasite) lives on or in another (the host) and feeds on it, harming the host. Example: a tapeworm in a human gut, a tick on a cow, or Plasmodium in humans.
• Mutualism: Both organisms benefit. Example: Rhizobium bacteria in the root nodules of legumes.

Step 3: Match the description to the definition

• "Lives on and feeds at the expense of the other" means the host is harmed while the parasite benefits.
• Commensalism does not fit because nobody is harmed.
• Predation does not fit because the predator does not live on its prey.
• Mutualism does not fit because both partners gain.
• Parasitism fits exactly.

Step 4: Answer The correct option is C. parasitism.

Exam tip: Use the "+/−" trick for symbiotic relationships:

• Mutualism = (+/+)
• Commensalism = (+/0)
• Parasitism = (+/−)

Whenever you see the words "at the expense of" or "harms the host", think parasitism. Also remember that a parasite usually lives with its host for a long time, while a predator kills quickly.