Several technologies in the research tree have one of two or three enhancements, of which you are only allowed to research one.  However, it is possible to obtain more than one through tech trading.  Some players consider all tech trading to be exploitive, and thus disable tech trading during game setup.  Even among players whom do not consider all tech trading to be exploitive, however, some might consider my further tactic below to increase the chances of getting all enhancements a bit exploitive; it is up to you.

If you trade other civs (including minor civs whom are generally slow at tech) the base tech (such as "Weapon Systems" above) and avoid trading them any enhancement techs you already have, they will then research one of the enhancement techs.  When you have a good pool of rival minor and major civs (the more civs the better, so ideally include minors), and you make sure all of them have the base tech by trading the tech to those that haven't researched it on their own, chances are good eventually each of the enhancements will be researched by at least one of those rival civs, so you can eventually trade for each of those enhancement techs and can wind up with all of them.

To really "min-max" this to maximize your chances of being able to acquire all enhancement options, use the following rules:

• Do not trade enhancement techs to AIs that haven't researched one of that enhancement's alternatives yet; once you or an AI has gotten an enhancement (even through trading), you aren't allowed to research an alternative -- and you want them to research an enhancement tech on their own to maximize their researching one you don't have.  (If you have all the enhancement techs yourself, of course, this rule is null and void)
• Similarly, don't trade enhancement techs from an AI when you haven't already researched one on your own.  Unfortunately, unless you have a photographic memory or every tech tree (if you aren't aware, there are several different tech trees depending on which race you play) printed out, you'll probably have to flip back and forth a lot between diplo screens and the tech tree to figure out which enhancement techs are alternatives to one another; often with the 3-option enhancements, one will rarely get researched by an AI as you may find checking for those enhancement techs' availability for trade in diplo screens; in such a case (if two of the enhancement techs have been researched by the AI civs, but one hasn't), the thing to do BEFORE trading for any of them is to research the one no AI has researched yet, THEN trade for the other two.  Remember that if you trade for any enhancement tech, you won't be able to research any of its alternatives on your own.
• Your chances of getting all enhancement techs are actually better if you are patient, trade away the base tech without researching the enhancement techs, wait for two of the enhancements to get researched by the AI civs and then (after finding two have been researched) researching the third on your own, and after researching the third one trading for the other two