I chose "other" because I always play with small parties. Because of my location and a general lack of gamers in the area I rarely have a campaign featuring more than two players. Usually, it's just one. So, if one person cancels...it generally means the game is cancelled. D&D Solitaire isn't much fun after all. Nonetheless, I'm in that small group of individuals who very much prefers small numbers of players. So, if I was running a group with say four people and only one showed up, I'd probably be more comfortable running a session with that one individual. If the group was tied up in the middle of something that made the others necessary, I might play a "flashback" for the remaining player's character, filling in his/her background.
As for NPCs, my games are awash with NPCs. I've created hundreds of them in my most recent campaign and when the PC(s) travel to most cities there's someone they know or have met before ready with a name and history. There are also quite a few adventuring NPCs to round out the party. This leaves the PC(s) free to be central to the story and never leaves a player feeling slighted because the focus of the game is elsewhere.