Charles Darwin and Francis Darwin discovered that the phototropic stimulus is detected at the tip of the plant. In their experiment, they used canary grass which was protected by coleoptile. After sometime, the coleoptile stoped growing and the primary leaf comes out of it. Hence, it was discovered that the tip of the coleoptile was the stimulus for phototropism and bending towards light, answer is b)phototrophism

Phototropism in Coleoptile Growth:

Phototropism is the growth of an organism in response to light. In the case of the coleoptile of canary grass, it exhibits growth towards the light source due to phototropism. Here's why:

Unilateral Illumination:
- When the coleoptile is exposed to light from one direction, it responds by growing towards that light source.
- This phenomenon is known as unilateral illumination, where the plant exhibits growth in a specific direction in response to light.

Phototropism:
- Phototropism is the specific growth response of plants towards a light source.
- The coleoptile of canary grass shows phototropism by elongating towards the light to optimize photosynthesis and maximize light absorption for energy production.

Photoperiodism and Geotropism:
- Photoperiodism is the response of plants to the length of day and night, regulating processes like flowering.
- Geotropism is the growth response of plants towards gravity, such as roots growing downward and stems growing upward.
- While these processes play essential roles in plant growth and development, they are not directly responsible for the coleoptile's growth towards the light source.

In conclusion, the coleoptile of canary grass exhibits growth towards the light source due to phototropism, a specific plant response to light that allows for optimal light absorption and photosynthesis.