I planted some fruit seeds last year, but I cannot recall whether they were mangosteen, pomelo, pomegranate, plum or something else.
I tried to compare the leaves of against some online images but I am still confused as what this seedling could be.
Click on photo for full size
There is a possibility that this is a pomegranate seedling. If it's not that, it must be something you didn't plant.
This does not readily appear to be any of the fruits listed in your question. It may be pomegranate, if the leaves are extremely sub-opposite. The leaf shape, margin, color, size, and pubescence fit with pomegranate. It could be a weed, or a volunteer species that you didn't mean to plant, or may have been planted by something else.
Here are the major points on why this isn't on the list you gave:
Mangosteen: The leaves are placed opposite each other. On your plant the leaves appear alternate, and circle the seedling.
Pomelo: These have winged petioles (see photo), which are unapparent on your plant.
Pomegranate: The leaves are placed opposite (or sub-opposite) each other. Your plant appears to have alternate leaves. This is only possibility I see here. The leaf arrangement might be extremely sub-opposite.
Plum: have leaves with serrated margins, and are pubescent. Your seedling has entire leaf margins, and the leaves are glabrous.
