Soldering / Brazing copper to stainless steel keg

[WCG]

Hi guys, have a pony keg and going the route of a 1500 watt hot water tank heater element. Picked up the element yesterday at Lowes (I am in Canada) along with a copper adapter that will work with the threads on the heating element.

To solder the copper adapter to stainless keg was curious what type of solder and flux I should buy. While I was in Lowes I snapped pictures of the different solder and fluxes they had in the plumbing section.

What they had was
1.) Rosin Core Solder - Lead Free - (Electrical)
2.) Acid Core Solder - Lead Free - (General Metal)
3.) Solid Wire Solder - Lead Free (Plumbing)
4.) Tinning Flux (for joining dissembler metals)
5.) Paste Flux (general purpose)
6.) Water Soluble Flux - (Plumbing)

Curious which would be the recommended solder and flux I should buy?

(Will need to post the pictures later, for some reason not able to get them off my phone and onto computer right now.)

Regards,
WCG