If you’re not familiar with SVN and merge resolution I suggest avoiding the advanced merge until you get comfortable with it.
To sum it up in a sentence: Both of the first two options should be suitable if your scenario is simply branch out and back into mainline, use Advanced merge if you’re familiar with SVN or if you’re doing something a bit more complex such as merging revisions from Mainline to the branch or maintaining multiple branches at once.
Merging can easily result in errors and there is no one way to go about it, you’ll have to determine yourself which version of an item is correct between what’s on mainline and what’s in your branch and often the answer can be somewhere in-between so you will have to work through your errors.
Here’s a bit of documentation from Mendix on the options you have: https://docs.mendix.com/refguide/merge-dialog#1-introduction
Hope this helps
Might be wise to first merge the Mainline into your Branch and solve the conflicts there.
Then resolve the conflicts in that branche.
Then merge the branche to the Mainline.