Which of the following is a natural defense mechanism of a plant against herbivores?

A. Producing toxic chemicals
B. Nuclear fusion
C. Developing large petals
D. Changing color according to temperature

The correct answer is A. Producing toxic chemicals.

To determine the natural defense mechanism of a plant against herbivores, we need to understand the characteristics of various defense mechanisms and identify which one fits the description. Let's discuss each option:

A. Producing toxic chemicals: Many plants produce toxic chemicals to deter herbivores from feeding on them. The toxicity of these chemicals can range from mild to severe, and they can cause digestive distress or even death in herbivores that consume them. This defense mechanism allows plants to protect themselves against being eaten.

B. Nuclear fusion: Nuclear fusion is a process that occurs in stars, not in plants. It is not a defense mechanism against herbivores.

C. Developing large petals: Large petals are often associated with attracting pollinators, such as bees and butterflies, rather than defending against herbivores. This option does not describe a defense mechanism.

D. Changing color according to temperature: While some plants may change color in response to temperature, it is not a defense mechanism against herbivores. Color change is often related to environmental factors, such as seasonal changes, and has different purposes, such as attracting pollinators or signaling the plant's life cycle stage.

Therefore, the correct answer is A. Producing toxic chemicals. Plants produce these chemicals as a natural defense mechanism to deter herbivores and protect themselves.