Option a is the correct answer. The Achaemenid conquest of the Indus Valley began in the 6th century BCE, with the first invasion led by Cyrus the Great. He annexed areas west of the Indus River, establishing control over northwestern regions of the Indian subcontinent, primarily in present-day Pakistan. Darius the Great later continued the campaign, expanding Persian territories in India up to the Jhelum River in Punjab around 518 BCE.