It's a matter of personal preference, except perhaps 3 which is just lazy and will eventually be inlined through editing. I think the
Direct linking to the study or article. is the best way to go for a number of reasons:
- Reference at the end of the post.
This option requires additional work to verify the claim. Answers should be thorough and they simply aren't if I have to go onto Google to check if your answer is correct.
It also breaks the flow of the post, because I either have to stop and look at the end of the post and return to my place, or remember what all the numbers are referring to at the end of the answer.
Another problem is whether you yourself are certain that the reference is accurate. If you have the book to hand and found a passage yourself, fair enough, but if you found the reference in a book, this reference is now second hand and you're relying on the intermediary author's ability to reference correctly. Granted it's unlikely, but it can happen, particularly in non-peer-reviewed materials.
- Throwing the link at the end of the paragraph.
This is quite simply ugly and lazy. The URL adds no real information and should just be embedded in a description of the page content (or the claim it is backing up).
If there actually is no link, then I'd go with Oddthinking's suggestion of a full Harvard style (or equivalent) reference directly in the text, though you should definitely quote the point verbatim in your post so people don't have to go to the local library to back up your claim or find out what you're referring to.