Step-by-step explanation:
the normal right answer is to make it hard for others to change the document, and every computer platform has a way to open PDF files.
so, let's see here what your teacher means by eliminating the clearly wrong answers :
A.
document properties are always included in the file by any word processor (and their file formats).
that is no reason to specifically use PDF.
C.
Mac D cask open also other file formats. no need to go specifically for PDF.
D.
a PDF file can also be opened by at least the ADOBE Reader. so, this is false anyway.
that leaves B. to be the right answer.
but I personally disagree with that as an exclusive criteria. there are other file formats that could be used for that too.