The correct option is A of
From the options, 'of' indicates a relationship between two things, here it expresses a relationship between the adjective "desirous" and the noun phrase "visiting Japan". Hence 'of' is the most suitable preposition and A is correct.
'In' refers to a place or a time, e.g, 'I am in Japan' or 'I reached in time'. It cannot be used here, as the sentence does not focus on place or time.
'To' expresses motion in a direction, for example, 'I am going to Japan'. But, the words "desirous...visiting Japan" do not speak in terms of direction.
'About' means 'on the subject of', 'with reference to', for example, 'a book about Japan'. It does not comply in meaning with the sentence.
Hence options B, C and D are incorrect.