What runes on Thror's map are visible only under moonlight?

The runes on Thrór’s map visible only under moonlight are moon-letters.

Moon-letters are a special form of hidden writing used by the Dwarves of Middle-earth. They cannot be read under ordinary light; they appear when the map is exposed to moonlight of the correct phase and season. This makes them both a message and a timing device, since the instructions depend on a particular lunar arrangement.

Elrond discovers the writing when Thorin’s company reaches Rivendell. He identifies the marks as moon-letters and reads instructions concerning the secret entrance to the Lonely Mountain. The message explains how the setting sun on Durin’s Day will reveal the keyhole in the otherwise invisible Back Door.

The other options are related Tolkien writing terms but are not correct here. Cirth is a runic alphabet associated especially with the Dwarves, while Tengwar is an Elvish script. Ithildin is a different magical, moon-revealed substance used in inscriptions such as the Doors of Durin. Moon-letters describe the hidden writing itself and its special method of revelation.