The correct answer is option 3: Mammal. This is because mammals have a higher sensitivity to environmental changes compared to other options listed.

Amphibians (option 1) are also sensitive to environmental changes, but they have the ability to adapt to a certain extent. They are often used as indicator species in environmental monitoring programs due to their sensitivity to pollution and habitat loss. However, mammals have a more versatile ability to adapt to various environmental conditions.

Reptiles (option 2) have a lower sensitivity to environmental changes compared to mammals. They are ectothermic, which means their body temperature is mainly regulated by the environment. While they can adapt to different temperatures, they generally have a more limited ability to cope with drastic environmental changes.

Insects (option 4) are highly adaptable and can survive in a wide range of environments. They have evolved to have various physiological and behavioral adaptations, allowing them to withstand environmental changes to some extent. However, mammals have a higher sensitivity to changes in their surroundings.

In summary, while all options listed have some level of sensitivity to environmental changes, mammals (option 3) are generally considered the most sensitive due to their ability to adapt to a wide range of conditions.