Speaker and amp pairing

b c are certainly correct, a is not wrong but not the right way to think about it

but there is more nuance and details to it than just that if you want to optimize the match between speaker and amp

You want to pair speakers that sound good with amps that sound good. Limit your speakers to at least 92dB sensitivity and just about any power will do.

Or you can spiral down the rabbit hole. Just know it goes so deep Arne Saaknussem gave up. 

Here’s how I think of speaker sizing and amp matching:

1. how big is the room - size the speakers accordingly
2. how loud do you like to listen - speaker sensitivity and thus amp power should be able to produce the desired db/SPL (can check with any calculator)
3. what’s the nominal impedance of the speakers’ bass woofers and midwoofers? This is a function of the size and number of the drivers, and the crossover topology. Generally, for a standard 2-way with 6.5" driver, the impedance will drop to 3-4ohms at its lowest points even if rated 8ohm nominal. If nominal impedance is 4ohms, well, it’s likely to drop even lower. Be very careful about the current demands for 4ohm nominal speakers - look at the amp design and specs and it should be easy enough to say a NAD 3020 or flea-powered tube amp is going to have a bad time.

Assuming there are no technical red flags, the rest is timbral matching and sonic preferences.

If you aren't absorbed into the tube vs solid state thing, just buy a larger 100W to 150W per channel solid state amp from a reputable audio brand and you should be okay.  Even new NAD and Yamaha larger amps sound okay vs older amps from 20 years ago.  A good 100W/ch amp will drive even lower sensitivity speakers.  Class A/B generally sounds better for the money, due to the larger linear power supply.  Usually lots of used integrated amps (preamp + amp together) for sale here which often offer larger output than receivers which can focus on features you may not need (bluetooth, airplay, etc).  A $500 used integrated from the last few years would be much better sounding than a new $500 (entry level) denon receiver.  
Brad 

There are lots of variables in play, but I learned a long time ago that not all watts are created equal, and that the measured specs for many amplifiers don't tell the whole story about how well they'll actually drive a speaker.  Nor the do specs indicate how an amp will sound.  Some of that is due to the fact that speaker impedances vary a lot, but most don't pose an unusually difficult load for a decent amp. 

If the nominal speaker impedance is rated at 8 ohms, most speakers will do fine with a well designed amp that has good current flow, unless the speaker is known to have a very low or complex impedance.  If nominal impedance is 4 ohms, you'll want an amp that control a low impedance speaker well.  It shouldn't be too difficult to research if any speakers you're interested in are known to to pose very low and/or difficult to drive loads, or how well a particular amp handles more difficult loads.  

As long as a few minimum impedance requirements are considered, I've always tended to focus more on how the speakers and amp sound together, with more focus on the speakers first, then try to find an amp that's capable and that I like the sound of.  It can take time, and trial and error, but can always bring a lifetime of enjoyment when done well.