Water is the second most important factor influencing the life of organisms because it plays a crucial role in their survival and functioning. Here is an explanation of why option D, which states that both (a) and (b) are correct, is the correct answer:

Water makes up a major part of an organism's body:
- Water is an essential component of living organisms, accounting for a significant portion of their body composition. For example, the human body is composed of about 60% water, while other organisms have varying water compositions.
- Water is a universal solvent that participates in many metabolic reactions within the body. It acts as a medium for transporting nutrients, hormones, and waste products within cells and throughout the body.
- It helps in maintaining the body's temperature through processes like sweating and evaporation, which aid in thermoregulation.

Productivity of plants depends upon the availability of water:
- Water is crucial for photosynthesis, the process by which plants convert sunlight into energy-rich molecules. During photosynthesis, plants absorb water through their roots and transport it to the leaves, where it combines with carbon dioxide to produce glucose and oxygen.
- Adequate water availability is necessary for the opening and closing of stomata, tiny pores on the surface of leaves, which regulate gas exchange. Stomata allow carbon dioxide to enter the plant and release oxygen and water vapor.
- Water also serves as a transport medium for nutrients and minerals absorbed by plant roots from the soil. It helps in their movement upwards, ensuring proper growth and development.

Life on Earth originated in water:
- It is widely accepted that life on Earth originated in water. The earliest forms of life, such as single-celled organisms, evolved in the Earth's oceans. Water provided a stable environment for the chemical reactions necessary for the formation of complex organic molecules and the development of early life forms.
- Even today, many organisms, including algae, bacteria, and various marine species, depend on water bodies for their survival. These organisms have adapted to their aquatic environments over millions of years.

In conclusion, water is the second most important factor influencing the life of organisms because it constitutes a major part of their body composition and is essential for their survival and functioning. Additionally, the productivity of plants, which form the basis of many food chains, relies on the availability of water. Furthermore, the origin of life on Earth is closely tied to water, making it a vital factor for the existence and evolution of organisms.