Explanation:
- The word "sustenance" refers to something that provides nourishment or support for survival.
- To find the word that names a necessary part of "sustenance," we need to identify a word that is closely related and essential for providing nourishment or support.
- Let's examine the options provided:

a) Nourishment:
- Nourishment is the process of providing or obtaining the food necessary for health, growth, and development.
- Nourishment is directly related to sustenance as it is the act or process of providing sustenance or the necessary nutrients for survival.
- Therefore, "nourishment" is a necessary part of "sustenance."

b) Water:
- Water is essential for sustaining life and is necessary for maintaining bodily functions.
- While water is a vital component of sustenance, it is not the only necessary part.
- "Water" alone does not encompass the full meaning of sustenance, as sustenance refers to a broader concept that includes nourishment from various sources.
- Therefore, "water" is not the correct answer.

c) Grains:
- Grains are a type of food that provides carbohydrates, fiber, and some essential nutrients.
- While grains can contribute to nourishment and sustenance, they are not the only necessary part.
- Sustenance encompasses a wider range of foods and nutrients necessary for survival.
- Therefore, "grains" are not the correct answer.

d) Menu:
- A menu is a list of food and drink items available for purchase or consumption at a restaurant or establishment.
- While a menu may include options for sustenance, it is not a necessary part of sustenance itself.
- A menu is a tool used to choose and order sustenance, but it is not an essential component.
- Therefore, "menu" is not the correct answer.

Conclusion:
- After analyzing the options, it is evident that the correct answer is a) Nourishment.
- Nourishment is a necessary part of sustenance as it encompasses the process of providing the necessary nutrients for survival.