The width of the letter or numeral in a registration number must be what proportion of the height?

The correct choice indicates that the width of the letter or numeral in a registration number must be twice the height dimension. This requirement is established by regulations that aim to ensure adequate visibility and readability of aircraft identification markings. When the width is twice the height, it maintains a balanced and proportional appearance, which enhances legibility from a distance.

This proportion helps in distinguishing the registration numbers clearly during inspections and enhances overall visibility, especially in flight operations. Such regulations are crucial for safety and compliance in aviation, allowing for easier identification of aircraft by authorities and other pilots.

In the context of the other potential answers, it's important to note that while specifying dimensions like two-thirds of the height, one-half the height, or specific measurements like twelve inches could provide various visual representations, they do not align with the established requirements for aircraft registration markings, which emphasize a wider proportion for enhanced clarity.