Answer:
Ruler
Step-by-step explanation:
A title bar is mainly located at the top of an application window. It mainly holds the application name and a few other information about the application. It could also provide special buttons for minimizing, maximizing, or closing the application. It does not determine if a line is truly horizontal or vertical.
A menu bar contains essential functions in an application such as opening, creating, editing and closing a file. It does not determine if a line is truly horizontal or vertical.
A ruler enables users of programs to align elements - graphics, texts, tables e.t.c - on a page. It contains two sections - the horizontal and the vertical. So, it can help to determine whether or not a line is truly vertical or horizontal.
Zoom allows elements on a page to appear bigger or smaller. It does not determine if a line is truly horizontal or vertical.
The best answer to the question therefore is, a ruler.
Hope this helps!