Final answer:
The highlighted part in TAB staff represents a fret, which is the correct answer to the student's question. Tablature is a form of musical notation for string instruments like the guitar, where the numbers indicate specific frets to be pressed on the strings.
Step-by-step explanation:
In TAB (tablature) notation for string instruments like the guitar, the highlighted part indicates a specific fret that needs to be pressed. TAB staff lines represent strings, and the numbers written on these lines signify which fret should be pressed down when playing that string. Therefore, the correct answer to the question would be A. a fret. Understanding how to read tablature is essential for guitarists as it provides a straightforward method of conveying music without the need to read traditional musical notation.