Flex takeoffs are not allowed in the following four conditions:

A. Wet runway, tailwind, contaminated runway, high altitude
B. Dry runway, headwind, uncontaminated runway, low altitude
C. Dry runway, tailwind, contaminated runway, high altitude
D. Wet runway, headwind, uncontaminated runway, low altitude

Flex takeoffs are not allowed in conditions that reduce safety margins, such as wet or contaminated runways, tailwinds, and high altitude scenarios. These factors negatively impact the aircraft's ability to safely accelerate and achieve necessary lift. The correct multiple-choice option is (a).

Explanation:

The conditions under which flex takeoffs are not allowed include: a) Wet runway, tailwind, contaminated runway, high altitude. These specific conditions present increased risks during takeoff for various reasons. A wet or contaminated runway reduces the friction between the aircraft's wheels and the runway surface, making it harder to accelerate safely. A tailwind reduces the effective airspeed over the wings at the start of the takeoff roll, potentially increasing the required takeoff distance. High altitude decreases air density, which diminishes engine performance and wing lift, making it harder for the aircraft to achieve takeoff speed.

Option b) is incorrect because it describes ideal conditions for a takeoff. Options c) and d) also mention conditions that are not typically prohibitive by themselves but, when combined with others in option a), can contribute to a decision against a flex takeoff. The correct multiple-choice option is (a).