Explanation:
The correct answer is option 'A' - Across.

Meaning of the Collocation:
The collocation "right across" means to be located directly on the opposite side of something.

Example Sentence:
His office is right across the road.

Explanation:
When we say that something is "right across" a road, it means that it is situated directly on the opposite side of the road. In this context, it indicates that the office is located on the other side of the road, facing the person who is speaking.

Collocation:
- "Right across" is a common collocation used to describe the position of something in relation to a road or street.

Alternative Collocations:
While the collocation "right across" is the most suitable answer for the given sentence, there are also other collocations that can be used to convey a similar meaning:

- "Just across" can also be used to describe the position of something in relation to a road or street. Example: His office is just across the road.

- "Directly across" can be used to emphasize that something is situated exactly on the opposite side of a road or street. Example: His office is directly across the road.

- "Opposite" can also be used to describe the position of something in relation to a road or street. Example: His office is opposite the road.

Conclusion:
In summary, the most suitable collocation to complete the sentence "His office is right ______ the road" is "across." This collocation accurately conveys the meaning that the office is situated directly on the opposite side of the road.