Let me add some additional challenges to the mix. I'm not sure what options might fall off the table based on this info. Aerial cabling from the house/router/isp may not be feasible. I believe that some of the greenhouses have limited A/C power that is likely brought in via underground pvc or metalic conduit. Not 100% sure! I did a quick distance calc from Google Earth. The house is approximately 681 feet from the farthest greenhouse. Will distance impact and or lack of AC power impact any of the other options?
700 ft is not a big deal for wired or wireless. just about every wireless solution has a 300 ft limit, because that is what the regulations are for unlicensed spectrum. but there are passive and active ways to extend the wireless, as well as ways to reduce the distance by clever placement of the base stations. 700 ft is easily do-able with one-wire, as long as you power it and don't have major rf interference in the area. 700 ft is easily do-able with ethernet too - you can run power over ethernet if you do not already have power in the greenhouses. unless you've got a lot of granite to deal with, i would run the wires underground in pvc conduit rather than hanging them. but whether you go wired or wireless, make it critter-proof (6-legged, 8-legged, and 4-legged).
Based on ease of set up and the UI, so far I'm leaning toward option a but am open to all other options.
since you will be supporting it, your level of comfort with the solution is probably most important metric.
every solution in the list will require some scripting. none of them require programming, although the one-wire solutions could (depending on which toolchain you choose).
so are you happier using a cloudy solution like ifttt? or do you prefer a self-contained ide that you own end-to-end? do you mind saving the data in the cloud? is the history as important as the notifications? is this a one-off, or do you plan to integrate it with irrigation, ventilation, or other systems? how long until you get fed-up with the canned solutions provided by the cloud providers and need your own displays and notification systems?
even the most expensive solution (wireless tags) is not that expensive; you will spend much more time on this than you will money.
If necessary, are there folks here on WXForum who we could contract the coding to for a fee?
there are already a number of open source solutions that would work almost right out of the box - probably on the order of a day or three of scripting and html level work. most of the time would be spent doing your dashboards and notification, and that is true whether you go with a canned cloud config or something bespoke.
perhaps someone more familiar with this forum could say what the policies/practices are here for soliciting work?