I could be wrong here, but "option A" generally requires the use of "option B." The sling swivels (I will call them "gate style" for the lack of a better term) seen at the very bottom of the pic in the second link are generally meant to attach to sling studs, such as those pictured in the first link you posted (option A). Usually, the sling studs are included with the shotgun.
Now, with my Benelli (for instance) the barrel retention ring has an eyelet drilled into the ring itself. In addition, the Comfortech stock has an eyelet molded into the stock. The SteadyGrip (pistol grip) stock I own has a sling plate. It all depends on the gun.
Regardless, the sling stud is essentially a "base" upon which various swivels attach. Unless (when considering the Mossberg) you have a picatinny rail on your gun or choose to use a sling plate, you will probably need a sling stud (possibly two if a sling "eyelet" is not molded into your guns stock). In terms of the sling stud(s), I suggest that, upon installation, you use LocTite Blue in order to secure these (as is my personal preference).
Other options would be to buy a sling mount (sling plate?) which goes between the mag tube and mag/barrel retaining cap. This particular type of mount is also made for installing between the receiver and the stock. These two mounts are good for a 1-point/2-point conversion-type sling (yes..tactical). Look to GG&G, Scattergun Technologies or possibly Mesa Tactical for such mounts. Personally, for the Mossberg (I have a 590), I prefer the GG&G brand.
If you simply want to mount a sling swivel (bottom of pic 2) to the sling swivel stud (pic 1) for a basic 2-point setup, Uncle Mikes makes a decent unit. Also, GG&G might make such an item. Generally speaking, these two items should be easy to find. Sling studs and sling swivels are plentiful and generally quite inexpensive. Also, some slings actually come with the sling swivel already attached (thus only necessitating the need for studs).
Many options exist and therefore, what type of sling you choose to go with will determine what is/what is not necessary for mounting such a sling.
By the way,
what model of Mossberg do you have? This may help in determining the most suitable options.
Please feel absolutely free to PM me if you have further questions or are interested in any links. I will try to help you the best I know how.
Im sorry if I have confused you, but slings and sling mounts can be as confusing and varied as options on automobiles. Best thing is, as I mentioned before, decide on what your needs are, find a good sling and then find the appropriate mounting solution. Its as simple (and yet, confusing) as that!