Explanation:

- The Armenian language contains multiple dialects.

- These dialects can be vastly different from each other.

- Option 1: understood - This is the correct word. It means that speakers of some dialects may not be able to comprehend the speech of those using other dialects.

- Option 2: realised - Not suitable. It refers to becoming aware of something, which isn't applicable here.

- Option 3: memorised - Incorrect. It suggests learning something by heart, unrelated to linguistic comprehension.

- Option 4: completed - Irrelevant here. This implies finishing something, which doesn't fit in the context of dialect comprehension.