When should minimum separation of 3 nautical miles be increased due to restrictions?

The correct choice is indicated as when aircraft are not in visual contact, and this is indeed a crucial factor in determining the minimum separation distance. When aircraft are not in visual contact, it increases the risk of miscommunication and potential collisions, prompting the need for increased separation to ensure safety. In such scenarios, maintaining the standard minimum separation may not be sufficient to account for the uncertainty in positioning or the actions of the pilots, necessitating a larger buffer zone.

In contrast, while flying below FL100 may influence operational procedures, it does not inherently dictate a need for increased separation on its own. Similarly, although deteriorating weather conditions may impact visibility and can lead to adjustments in separation standards, the critical aspect leading to increased minimum separation relates primarily to the loss of visual contact. Lastly, the consideration of whether aircraft are operated by different airlines does not inherently require adjustments to minimum separation distances, as safety protocols apply uniformly regardless of airline affiliation.