Which of the following statements holds true for the term spider?

a$.$

It refers to the process of improving a page's organic search results.

b$.$

It refers to a temporary storage space used to speed computing tasks.

c$.$

It refers to the Internet directory service that allows devices and services to be named and discoverable.

d$.$

It refers to software that traverses available Web links in an attempt to perform a given task.

e$.$

It refers to Internet content that can't be indexed by Google and other search engines.