Your question is incomplete because you have not provided the answer options, which are:
A. The capital of Japan-I have read that this huge city has over 30 million people-is Tokyo.
B. The capital of Japan, I have read that this huge city has over 30 million people, is Tokyo.
C. The capital of Japan-I have read that this huge city has over 30 million people is Tokyo.
Answer:
A. The capital of Japan-I have read that this huge city has over 30 million people-is Tokyo.
Explanation:
The use of dashes suggests that there is a sudden change of thinking, an idea that is not continued or a recapitulation of the topic. In that respect, the chosen sentence is the only one that makes use of dashes correctly, to open and close the unexpected changing of topic. Meanwhile, the use of commas is used to add extra information, as in non-defining and non-restrictive relative clauses - not to indicate an abrupt transition to a different topic.
