Option d is the correct answer. Mahajanapadas were ancient kingdoms or republics that emerged in the Indian subcontinent around the 6th century BCE. The exact number and composition of Mahajanapadas can vary based on different historical sources, but Buddhist and Jaina texts generally list sixteen Mahajanapadas. Option a is incorrect: While some Mahajanapadas may have been oligarchies, others had different forms of governance, including monarchies and republics. Option b is incorrect: Mahajanapadas were spread across different regions of ancient India, including both eastern and western parts of the Indian subcontinent. Option c is incorrect: Mahajanapadas had their own armies to protect their territories and engage in warfare if necessary.