Q.Select the statement which explains best parasitism.

Parasitism is an interaction where one species (the parasite) derives nourishment and shelter at the expense of another species (the host). The parasite benefits by obtaining resources it needs to survive and reproduce, while the host is harmed because the parasite feeds on its tissues, blood, or nutrients. This harm can range from reduced fitness and vitality to serious disease and even death in severe cases.

The key feature that distinguishes parasitism from other interactions is this one-sided benefit coupled with harm to the other partner. It differs from:

• Mutualism, where both organisms benefit
• Commensalism, where one benefits and the other remains unaffected
• Competition, where both are harmed

Examples include ticks feeding on mammals, tapeworms living in intestines, or the Cuscuta plant (dodder) parasitizing host plants. In every case, the parasite gains while the host suffers a cost. …

Ecological interactions shape the structure of communities and determine how species coexist, compete, or exploit one another. Among the various types of interactions—mutualism, commensalism, competition, predation, and parasitism—each is defined by the costs and benefits experienced by the organisms involved.

Parasitism represents a specific kind of relationship where the balance is distinctly one-sided. The parasite derives nutrition, shelter, or some other resource from its host, gaining a clear advantage. The host, meanwhile, suffers harm. This harm can take many forms: reduced growth, weakened immunity, loss of reproductive capacity, or in severe cases, death. The key feature is that the host is negatively affected, not merely left unchanged.

Consider the examples NCERT provides: the cuckoo bird lays its eggs in a crow's nest, and the crow unknowingly raises the cuckoo chick. The cuckoo benefits (it doesn't expend energy on parental care), while the crow is harmed (it invests resources in raising another species' offspring at the cost of its own). Similarly, parasitic plants like Cuscuta (dodder) extract nutrients from their host plants, weakening them. Human parasites—liver flukes, tapeworms, Plasmodium (malaria), roundworms—all thrive by exploiting their hosts, causing disease and debilitation.

Now let's evaluate each option:

• (a) One organism is benefited — This is incomplete. It tells us nothing about what happens to the other organism. Parasitism isn't just about one benefiting; it's about one benefiting while the other suffers. …

Eliminate options using the sign rule: rule out any option that names an effect for only one species (rules out a), then match the remaining three against …