The correct option is D. Association between two similar species. Detailed Explanation: Understanding Symbiosis: Symbiosis is a biological term that describes a close and long-term interaction between two different biological species. This relationship can take various forms, including mutualism, commensalism, and parasitism.

1. Option A: Symbionts must be living
2. This statement is true. In symbiotic relationships, the organisms involved (symbionts) must be living entities. Symbiosis inherently involves interactions between living organisms, such as plants, animals, fungi, or bacteria.
3. Option B: It is an association of 'give and take'
4. This statement is also true. Symbiosis often involves a mutual benefit where both organisms gain something from the relationship (as seen in mutualism). Even in other forms of symbiosis, such as commensalism (where one benefits and the other is neither helped nor harmed) or parasitism (where one benefits at the expense of the other), there is still an interaction that can be described as a form of 'give and take' in terms of resources or benefits.
5. Option C: The association may involve two plants
6. This statement is true as well. Symbiotic relationships can occur between two plants. For example, mycorrhizal fungi form symbiotic associations with the roots of many plants, enhancing nutrient uptake for the plant while receiving carbohydrates in return.
7. Option D: Association between two similar species
8. This statement is NOT true. While symbiosis can occur between different species, it does not necessarily imply that the species involved are similar. In fact, symbiosis typically involves different species. For example, a classic example of mutualism is the relationship between bees (insects) and flowering plants, where bees pollinate the plants while obtaining nectar. If two similar species were to interact, it would not typically be classified as symbiosis but rather as competition or other forms of interaction.

Why Other Options Are Correct:

• Option A is correct because symbionts must be living; non-living entities cannot engage in biological interactions.
• Option B is correct because symbiosis often involves a reciprocal relationship, where both organisms benefit in some way.
• Option C is correct because symbiosis can indeed involve two plants, as seen in various ecological interactions.

Common Pitfalls:

• Confusing symbiosis with competition: While both involve interactions between species, competition refers to the struggle for resources, not a mutually beneficial relationship.
• Assuming that symbiosis only occurs between different species: While it is most commonly between different species, there are instances of symbiotic relationships within the same species, but these are less common and often not classified as traditional symbiosis.

Revision Summary:

• Symbiosis involves living organisms and can take various forms (mutualism, commensalism, parasitism).
• It is characterized by interactions that can be beneficial to one or both parties.
• Symbiotic relationships can occur between different species, including plants and animals.
• The statement about symbiosis involving two similar species is incorrect, as it typically involves different species.