Behind the veil means hidden from public view or not openly known. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The documentary looked behind the veil of political power.
• In an HSC essay, Behind the veil can express this meaning: hidden from public view or not openly known.
• When this expression appears in a passage, connect Behind the veil with the idea of hidden from public view or not openly known.