Question

is the group of words in bold a phrase or a clause?
the most successful advertising slogans are those that have the ability to endure, such as \just do it\ or \when you care enough to send the very best.\

Explanation:

A phrase is a group of words without a subject - verb combination, while a clause has a subject and a verb. The bolded group "the most successful advertising slogans" lacks a verb (it has a noun "slogans" and modifiers but no action - performing verb and subject - verb relationship), so it is a phrase.

Answer:

The group of words in bold is a phrase.