**The pigment involved in red far red light interconversion is Phytochrome.**

**Phytochrome:**

Phytochrome is a type of photoreceptor pigment found in plants that is involved in various light-dependent processes, including the regulation of plant growth, development, and photomorphogenesis. It is a chromoprotein that absorbs and detects red and far-red light wavelengths.

**Red-Far Red Light Interconversion:**

Plants have the ability to sense and respond to changes in the light environment. The red-far red light interconversion refers to the conversion of light from the red spectrum (wavelengths around 660 nm) to far-red spectrum (wavelengths around 730 nm) and vice versa. This conversion is important for plants as it provides them with information about the quality and quantity of light available for photosynthesis and growth.

**Role of Phytochrome:**

Phytochrome acts as a light sensor and mediates the red-far red light interconversion. It exists in two interconvertible forms: Pr (red-light absorbing form) and Pfr (far-red-light absorbing form). When phytochrome absorbs red light, it is converted from the Pr form to the Pfr form, and when it absorbs far-red light, it is converted back to the Pr form.

**Mechanism of Red-Far Red Light Interconversion:**

1. Absorption of Red Light: When phytochrome absorbs red light, it undergoes a conformational change from the Pr form to the Pfr form.
2. Absorption of Far-Red Light: The Pfr form of phytochrome absorbs far-red light and undergoes a conformational change back to the Pr form.
3. Signal Transduction: The interconversion between Pr and Pfr forms triggers a signaling cascade within the plant cells, leading to various physiological responses, such as seed germination, stem elongation, and flowering.

**Importance of Red-Far Red Light Interconversion:**

The red-far red light interconversion is crucial for plants to adapt to changes in the light environment. It helps plants to determine the presence of neighboring vegetation, shade, and the quality of light available for photosynthesis. By perceiving the ratio of red to far-red light, plants can adjust their growth patterns, optimize light absorption, and allocate resources for survival and reproduction.

In conclusion, phytochrome is the pigment responsible for the red-far red light interconversion in plants. It plays a vital role in mediating plant responses to changes in the light spectrum, enabling them to optimize their growth and development.

Option c) is correct answer.