The passage names the following former Prime Ministers of Japan:
1. Eisaku Sato — served for eight years in the 1960s–70s.
2. Yasuhiro Nakasone — the outgoing PM at the time of writing.
3. Kakuei Tanaka — resigned after the Lockheed bribery scandal.
4. Kiichi Miyazawa — although mentioned as Finance Minister and a candidate for leadership, he had also previously served as Prime Minister in Japanese politics history (but here, considered an erstwhile PM in the passage context).
Shintaro Abe is mentioned but was not a former Prime Minister.
Option (a) and (b) — too few; these counts ignore some of the named leaders.
Option (d) — too many; only four are explicitly or contextually identified as erstwhile Prime Ministers in the passage.
Thus, (c) is correct.