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."

The group of words in bold is a phrase.

Explanation

Phrases are those sentences in which there are no verb and subject while clause are those set of words in which there are both subject or verb in them.The clause is generally used as a single part of speech. The phrase is a collection of words combined as one single unit.In the sentence "Just do it", there is a verb but no subject in it. Therefore, it is the phrase . On the other hand the sentence, "When you care enough to send the very best", "you" is  the subject here while there are two verbs "send" and "care" which makes it a clause.

Because writing on paper is said to engrave what your writing down into your brain therefore writing your thoughts is better than speaking them.

It could be 1, 2, or 4, depending on the context.1.) they could have had a fight, and she is too humble to say anything further. she wants robert to say something first.2.) maybe he's staring at a computer playing tetris while she's wearing a *really* tight red dress. just sayin' xd3.) is unlikely given that it's only one sentence worth of context long. 4.) it's possible their relationship is more 'mutual hatred' and they tolerate one another.it could be 1, 2, or 4, as long as you can provide sufficient explanation why. remember this as you write!