To understand why trailers and semi-trailers with an MMA over 750 kg require a registration certificate, it is important to know some details about vehicle documentation.

- **Registration certificate**: This document is compulsory for certain types of vehicles. In the case of trailers and semi-trailers, only those that can carry more than **750 kilograms** must have it. This means that if a trailer or semi-trailer has an **MMA (Maximum Authorised Mass)** over 750 kg, it needs a registration certificate.

- **Details on the registration certificate**: This document includes important information such as the holder’s name, the registration number, and the **maximum weight the vehicle can carry**. The latter is crucial to determine whether a trailer or semi-trailer needs the certificate, since it is based on its load-carrying capacity.

Therefore, only trailers and semi-trailers with an **MMA over 750 kg** are required to have a registration certificate, which makes the correct answer option 1.