Explanation:

To find the word that names a necessary part of the underlined word, we need to first understand the meaning of the underlined word.

The underlined word is "Desert." A desert is a barren area of land where little rainfall occurs and, consequently, living conditions are extremely harsh.

Now, let's look at the options:

a) Cactus: A cactus is a type of plant that is well-adapted to survive in arid environments. While cacti are commonly found in deserts, they are not a necessary part of a desert. Therefore, option A is not the correct answer.

b) Arid: "Arid" is the correct answer. The word "arid" means extremely dry or having little to no rainfall. This is a necessary characteristic of a desert. Without a lack of rainfall, a place cannot be classified as a desert. Therefore, option B is the correct answer.

c) Oasis: An oasis is a fertile spot in a desert where water is found, often surrounded by vegetation. While oases can be found in deserts and provide a source of water and relief, they are not necessary for a place to be considered a desert. Therefore, option C is not the correct answer.

d) Flat: The word "flat" does not directly relate to the concept of a desert. It refers to a surface that is level, smooth, or without curves or bumps. Therefore, option D is not the correct answer.

In summary, "arid" is the word that names a necessary part of the underlined word "desert." A desert is characterized by its arid conditions, meaning it has little to no rainfall.