The line of sight range from FL350 is about 235NM so you're not going to be able to use VHF. This suggests you use HF. The 'at night' bit is trying to suggest you should use lower HF frequency rather than a higher one.

Typical night time frequencies over the North Atlantic are around 3MHz, that's to say 3000KHz, and daytime frequencies are in the 9MHz to 11MHz range.

The best answer seems to be 5649Khz or 5.649MHz, if only because it’s the lowest one given.