Explanation:

- Megaliths are large stone structures created in prehistoric times.

- The practice of erecting megaliths began several thousand years ago.

- Many megaliths date back to the Neolithic and early Bronze Age periods.

- Option 1: 2500 years ago is too recent for the start of megalith construction.

- Option 2: 1000 years ago is far too recent as well.

- Option 3: 1500 years ago doesn't align with historical data about megaliths.

- Option 4: 3000 years ago is more accurate, though still conservative.

Option 4: 3000 is closest to the period when megaliths first began to appear, which aligns more correctly with the general timeline of 5000-7000 years ago.