Mumbai being close to the sea is affected by the sea breeze and has moderate type of climate throughout the year, whereas Ahmedabad is located far away from the sea and hence has extreme type of climate. So it is hotter in Ahemdabad in March as compared to Mumbai.
The real reason is-

Mumbai is in Maharashtra and Maharashtra is a country near the sea and cold breezes flow into the city making the climate colder.

While, Ahmedabad is in Gujarat and Gujarat is near the Equater. And we know that countries,states etc near the Equater are warmer.

this is why Ahmedabad has hotter climate.

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