Explanation:

- Tall gateway: This is the correct meaning of 'Pishtaq' in the context of medieval Indo-Islamic architecture. It refers to a frontal element or a grand arched gateway that leads into a building, often more pronounced in mosques and tombs.

- Water tank: Typically, in Indo-Islamic architecture, water features were significant, but 'Pishtaq' is not associated with water tanks.

- Dome: Domes are a prominent feature in Islamic architecture, symbolizing the heavens, but a ‘Pishtaq’ is not a dome.

- True arch: While Indo-Islamic architecture often employed true arches, especially in their gateways, a 'Pishtaq' refers specifically to the gateway structure itself, not just the arch.