There has been a lot of discussion over this question – We will try to explain it in line with the regulation.

EASA seems to distinguish between flights enroute (Cruising levels) and flights in the vicinity of aerodromes. In this case, the regulation that concerns us is the one related to “Cruising levels”.

• heights, for flights below the transition layer. - WRONG. The regulation does not refer to “heights”, only “altitudes” and “flight levels”
• altitudes, for flights at or below the transition altitude. - CORRECT. See regulation below.
• altitudes, for flights at or below the lowest usable flight level. - WRONG. What makes this option incorrect is the fact that it states “AT OR below”, when the regulation only says “BELOW”.
• flight levels, for flights in and above the transition layer. - WRONG. See regulation below.

Rules of the air, annex 2
CHAPTER 3. GENERAL RULES
3.1.3 Cruising levels

The cruising levels at which a flight or a portion of a flight is to be conducted shall be in terms of:

a) flight levels, for flights at or above the lowest usable flight level or, where applicable, above the transition altitude;
b) altitudes, for flights below the lowest usable flight level or, where applicable, at or below the transition altitude.

Note.— The system of flight levels is prescribed in the Procedures for Air Navigation Services — Aircraft Operations (Doc 8168).

ICAO Doc 4444

1- For flights in the vicinity of aerodromes and within terminal control areas the vertical position of aircraft shall, except as provided for in 4.10.1.2, be expressed in terms of altitudes at or below the transition altitude and in terms of flight levels at or above the transition level. While passing through the transition layer, vertical position shall be expressed in terms of flight levels when climbing and in terms of altitudes when descending.

Note: We are aware that some databases may have a different correct option, which is as follows: “altitudes, for flights below the lowest usable flight level”. We do not have that option, instead we have one which states: “AT OR Below” => making it incorrect.