The correct answer is option 2: Respect for individual rights.
Strong military forces (option 1) do not necessarily guarantee the development of democratic institutions. While a strong military may provide security and stability, a democracy requires more than just military strength.
A one-party system (option 3) is also not conducive to the development of democratic institutions. In a one-party system, there is usually limited political competition, which undermines the principles of democracy, such as pluralism and the protection of minority rights.
An agricultural economy (option 4) may or may not be relevant to the development of democratic institutions. Economic factors can certainly play a role in shaping political systems, but an agricultural economy alone does not determine the presence of democratic institutions.
Therefore, the most essential factor for the development of democratic institutions is respect for individual rights (option 2). Democratic institutions are built on the foundation of protecting individual freedoms, such as freedom of speech, assembly, and association. Without respect for these rights, a true democracy cannot thrive.