To identify the word that names a necessary part of the underlined word, let's analyze the options provided:

a) School
b) Teacher
c) Textbook
d) Learning

The underlined word in the question is "knowledge." Knowledge is a broad term that refers to the understanding, information, and skills acquired through experience, education, or training. In order to gain knowledge, certain elements are required. Let's examine each option to determine which one is necessary for acquiring knowledge:

a) School: While schools provide an environment for learning and acquiring knowledge, they are not an essential component. Knowledge can be obtained outside of the school setting.

b) Teacher: Teachers play a crucial role in imparting knowledge and guiding students in their learning journey. However, knowledge can still be acquired through other means without the direct involvement of a teacher.

c) Textbook: Textbooks are common tools used in formal education to provide information and guidance. While they are helpful in the acquisition of knowledge, they are not an absolute necessity. Knowledge can be obtained through various sources other than textbooks.

d) Learning: The word "learning" encompasses the process of acquiring knowledge. Without learning, knowledge cannot be gained. Learning involves the active engagement of individuals in acquiring new information, understanding concepts, and developing skills. It is an essential prerequisite for acquiring knowledge.

Therefore, the correct answer is option d) Learning, as it is a necessary part of knowledge acquisition.