The Taj Mahal is topped ----- a large dome.

The best preposition to describe the Taj Mahal's dome is: by.

Here's why:

• By indicating location or position about something else. In this case, the dome is situated on the top of the Taj Mahal.
• Over suggests being above something with some space in between. While the dome is certainly above the rest of the structure, "by" is more precise for its direct placement.
• Around implies circling or surrounding something. The dome isn't wrapped around the Taj Mahal but rather sits on its highest point.
• At indicates a specific location or point in time. It wouldn't be appropriate to describe the relative position of the dome.

Therefore, "by" most accurately describes the dome's position directly on top of the Taj Mahal.