I really don't expect it's the first option, unless they're really in a race to the bottom. The third option makes sense even without subsidies - offer something "nice" to typical customers and offset it with other traffic.
There is a difference to people who actually care a lot about how much they spend. A deal with FB may mean a local cache/interconnect and actual cheaper prices. Making other things more expensive because you're just trying to differentiate from competition is a money grab.
In a perfect world - yes. But phone/network providers have some weird tendency to end up being mono-/oligopolies. They like when prices are high at every provider, even without "real collusion".
For example look at roaming rates in Europe. For decades sending messages/data from another country was crazy expensive - with every single provider. Once EU ruled that roaming charges in the EU should not apply, there was no crisis or huge rise of prices to compensate the losses. Once it was forced, companies started to race each other to announce how awesome they are, being the first in the country to provide free roaming.
There is because it's quite different if you get one thing cheaper than normal, or if you get another more expensive than normal. Imagine next time you climb into a taxi the driver says "just for you I'll charge 20% extra". Pretty sure it will be different from hearing that somebody else got a 20% discount.
So making WhatsApp cheaper than the base price is one thing. Making Signal more expensive than the base price is quite different.