Hello. I will really appreciate some help from a native speaker of English.

Which word is correct in the following sentence: Both countries claim the four islands WHICH or THAT are known as... Maybe, both?
And one more question.
Is it possible to say: The islands are a (the ?)reason (cause) of dispute. OR The islands are a moot point OR a debatable point / issue. Does it sound natural?
Thank you very much for your time and help.

In the first sentence, you can use either WHICH or THAT.

If you say the islands are the reason for the dispute, then the islands are the only reason. If you use a reason, then there are other reasons in addition to the islands.

Your other sentence is correct, with any of your words.

Hello! I'd be happy to help you with your questions.

Regarding the first question, both "which" and "that" can be used in the sentence "Both countries claim the four islands which/that are known as...". Both are relative pronouns used to introduce a restrictive clause, providing additional information about the four islands.

To determine which one to use, you can consider the overall structure and meaning of the sentence. If the information in the restrictive clause is essential to understanding which four islands are being claimed, you can use either "which" or "that." For example: "Both countries claim the four islands that are known as...". Alternatively, if the information in the restrictive clause is just providing additional non-essential information, using "which" would be more appropriate. For example: "Both countries claim the four islands, which are known as...".

For your second question, all the options you provided can work in certain contexts. It depends on the specific meaning and tone you want to convey:

- "The islands are a reason (cause) of dispute." This sentence is grammatically correct and conveys that the islands are a source or cause of disagreement between the countries.

- "The islands are a moot point." This phrase means that the islands are a topic or issue that no longer holds practical significance or is open to discussion.

- "The islands are a debatable point/issue." This phrase implies that the islands are a subject or matter that can be argued or discussed.

All three options can sound natural depending on the context in which they are used.