The correct answer is D. to all without exceptions.
The phrase “across the board” means “to all without exceptions.” In this case, it means that all employees were given promotions, regardless of their performance or seniority.
Option A is incorrect because it refers to people sitting diagonally opposite each other. This is not what the phrase “across the board” means.
Option B is incorrect because it refers to people working for other boards. This is not what the phrase “across the board” means.
Option C is incorrect because it refers to people being given promotions selectively, with a few exceptions. This is not what the phrase “across the board” means.