Translation:The height of the lighthouse in Vlieland is seventeen meters.
'...IN Vlieland is...' ? - being an island, should it not be '....ON Vlieland is...' ?
Vlieland is not only an island, it's also a municipality, so both are accepted.
The municipality is Vlieland: https://almanak.overheid.nl/31502/Gemeente_Vlieland/
Oost Vlieland is part of the municipality Vlieland.
Other than all of this island/municipality thing. It would be very cruel to expect people learning Dutch to know that Vlieland is an island. So even if it wasn't a municipality it would make sense to accept in here, even if it wasn't technically correct. Although, here it is.
Cruel as it may be, the following information from: http://www.vlieland-info.nl/Enginfo.html
" 3. HET DORP (The Village) The island of Vlieland has only one village: Oost-Vlieland, with approximately 1100 residents. The village contains 39 buildings classified as monuments. These have been carefully rennovated to maintain the mainstreet in its original state. "
Your point being? Vlieland is the municipality. Sure it has only one village now. It used to also have West Vlieland. If they decide to build another village it will also be part of the municipality Vlieland.
Honestly, look up as much as you want. It doesn't change the fact that the municipality is called Vlieland, as you can see on the government website I linked to before.
I'm obviously missing something because I thought the idea of comments and discussions is all about improving the Duo presentation, not about scoring points. Be that as it may, the fact is that we are repeatedly told in the lessons that Vlieland is an island (which is not cruel) and as such the correct answer would be that the lighthouse is ON the island. If you happen to be in the know, or you delve into the intricacies of Dutch civil administration, you can establish that Vlieland municipality exists and encompasses one village of about a 1000 souls, Oost-Vlieland (the municipality of Teschelling consists of about 15 villages). If you then, outside the framework of the lessons, assume that Vlieland refers to the municipality and not the island, IN would also be correct as an alternative answer.