Are these words a sentence or a sentence fragment?

Sitting right there at the table.

A. Sentence

B. Fragment

The words are the following:

B. Fragment.

They are a sentence fragment, since there is no subject to be found. Who is sitting at the table? (No answer: no subject) A sentence needs a subject and a verb. The words at hand only have the latter. In other words, they lack a full sentence structure, which makes them incomplete to be called that.

If it were a sentence, it would look like the following example:

John is sitting right there at the table.

John: subject (Who is sitting at the table? / Answer: John is.)

is sitting: main verb (to sit) conjugated in present continuous tense.

right there at the table: adverbial phrase of place (it does not belong to a basic sentence structure, that is, even if omitted, the sentence would still be a sentence).