Which of the following could indicate a dog's discomfort?

Flat ears can indeed indicate a dog's discomfort. When a dog's ears are laid back against its head, it may be expressing feelings of fear, anxiety, or uneasiness. This body language is often accompanied by other signs of stress or discomfort, such as trembling or seeking to avoid interaction. Understanding a dog's ear position is crucial in interpreting its emotional state, as it can provide significant insight into how the animal is feeling in a particular situation.

In contrast, relaxed posture, open mouth and panting, and tail wagging can have different meanings depending on the context. A relaxed posture typically suggests that a dog is comfortable and at ease. An open mouth and panting may indicate excitement or a need to cool down, instead of discomfort. Tail wagging can signal a positive emotional state; however, the position and speed of the wag can alter its meaning significantly, so it should be assessed alongside other body language cues.