Translation:Alys's wedding dress is very beautiful.
The alternative answer is wrong. In English an apostrophe and an s ('s) is NEVER EVER put behind an s.
This sentence is a zombie, I am afraid, so I cannot check it. What does the alternative answer say?
'New Hart's Rules' and 'The Economist Style Book' both say that Alys's is correct - a singular name ending in '-s' does take the apostrophe-s.