Urban Planning and Architecture:

• The Harappan cities, such as Mohenjo-daro and Harappa, exhibited advanced urban planning and architecture. Streets were laid out in a grid pattern, and buildings were constructed with standardized bricks.

Sophisticated Drainage Systems:

• Harappan cities featured sophisticated drainage systems, with well-planned and organized networks of sewers and drains. This reflects a high level of engineering and urban management.

Advanced Harappan Script:

• The Harappans developed a script that is found on seals and inscriptions. While the script remains undeciphered, its existence suggests a level of literacy and communication in the urban centers.

Economic Specialization:

• The cities had evidence of economic specialization, with different areas dedicated to specific activities such as manufacturing, residential areas, and public spaces.

Trade and External Contacts:

• The urban centers were connected to extensive trade networks, with evidence of trade links with regions as far away as Mesopotamia. This indicates a sophisticated economic system.

So, the correct option is [3].