I think which one to work on depends on your definition of success and the path you want to take to get there. Do you want to raise money and build a huge company? Do you want to bootstrap a small profitable company? Or do you just want to have fun making something and getting as many people as you can to use it?
a is nice because you can do a quick MVP to validate it. If it catches on, you can run with it, but you won't waste too much time if it falls flat. Remember though that dating apps have the chicken and egg problem, so you need a plan for how to seed it with early users. It also has the issue that it's pretty easy to copy on a technical level, so you need to build a great community and/or come up with some kind of network effect that's hard to replicate. The challenges of this one will be more on the community-building side than the tech-side.
b seems like you'd be too dependent on an outside platform. Plus if it caught on, FB could just build it themselves. It sounds pretty easy to build though, so that's good. If it got popular and had a lot of users, maybe you could think of some interesting directions to take it.
c sounds interesting. If it's well-designed and well-marketed, I could see something like that going viral. How to make money is a bit of a tricky question though. Who would pay for this? Perhaps you could monetize with ads, but you'd need massive numbers.
d sounds useful, but it also sounds like a substantial project. I'd do some tech exploration to make sure it's reasonably doable and also some market validation ahead of time, as I presume it would entail a big project. If it works well, it sounds like something you could definitely charge for. Tech challenge aside, it's probably the most straightforward idea to turn into a profitable saas business.
e - sounds like a fun project. I could see it taking off on some subreddits, but it might be hard to sustain people's interest. From a business-perspective, you'd very dependent on an outside platform. But like with b, if you're able to get a ton of sign ups, you could potentially take it in some cool directions.