In which block would you expect to find remarks such as minimum fuel?

The block where you would typically find remarks such as "minimum fuel" is Block 26. This block is designated for remarks and additional information related to the flight that does not fit in the other blocks. It serves as a space for important notes that need to be communicated to air traffic control, such as special handling requests or any condition that might affect the flight, including fuel status.

In contrast, the other blocks each have specific purposes. For example, Block 16 is generally reserved for information about the flight's cruising altitude, while Block 21 contains details pertaining to the estimate and flight plan time. Block 25 is used primarily for the transmission of the aircraft's type and registration. These specific functions make those blocks unsuitable for general remarks like "minimum fuel." Therefore, Block 26 is the correct choice for conveying crucial operational remarks during a flight.