You drink a glass of lemonade, but your body’s PH doesn’t change even though the lemonade is very acidic. This example of which characteristic of life? 1. Adapt to the environment
2. Maintain homeostasis
3. Acquire energy
4. Are immune to acid

The correct answer is - optogenetics.

Explanation:

Optogenetics is a technique that is used in examining the basis of smell, fear, and memory that involves the use of light to regulate cells in a biological tissue especially neurons, which are genetically altered to show light ion channels.

Channelrhodopsin-2 is the most commonly used ion channel for triggering the optogenetics. When a definite number of positive ions have to get through the cell membrane, it causes the neuron to fire reaching the threshold.

Thus, the correct answer is - optogenetics.