Living organisms are classified as those that have the characteristics of life, such as growth, reproduction, metabolism, response to stimuli, and the ability to adapt to their environment. Based on these characteristics, we can determine if an organism is living or non-living.

In the given options, the correct answer is option 'A', Mushroom.

Let's break down the characteristics of living organisms and see how mushroom fits into them:

1. Growth: Living organisms have the ability to grow and increase in size. Mushrooms start as small structures called spores and then grow into larger structures with the help of nutrients and favorable environmental conditions.

2. Reproduction: Living organisms have the ability to reproduce and create new individuals of their own kind. Mushrooms reproduce through spores, which are released into the environment and can grow into new mushrooms under suitable conditions.

3. Metabolism: Living organisms have metabolic processes that allow them to obtain energy from their surroundings and use it to carry out various functions. Mushrooms obtain nutrients by breaking down organic matter in their environment and use them for energy.

4. Response to stimuli: Living organisms can respond to changes in their environment. Mushrooms can respond to factors such as light, temperature, and humidity by adjusting their growth and reproductive processes.

5. Adaptation: Living organisms have the ability to adapt to their environment over time. Mushrooms can adapt to different environmental conditions and grow in various habitats, such as forests, grasslands, and even on decaying organic matter.

Based on these characteristics, it is clear that mushrooms possess the qualities of a living organism. They grow, reproduce, have metabolic processes, respond to stimuli, and can adapt to their environment. Therefore, the correct answer is option 'A', Mushroom.