well, Google Ads is safe for sure since it's Google's own product.
Few years ago when I last checked Adsense's ToS they allowed any source of traffic, but of course that the caveat was "as long as the traffic is quality", which they obviously never said what it means because they need to have reason to kick you out if they don't like you...
But the point is that you can use pretty much any source of paid traffic that is decently legitimate (non incentivized to click on ads that is), so you are probably looking at Facebook and Pinterest as legit sources of traffic. I know for sure that Pinterest traffic was very OK with google many years ago, but I can't speak about FB or other social media, despite of google's reassurance that they're ok with it.
What I can say for certain is that traffic exchanges, safelists, and Paid-to-Click / Get Paid To programs (like Donkeymails for example) are a big NO-NO for Adsense