In what way did the building of the Taj Mahal contribute to the decline of the
Mughal Empire?

It Depleted the Mughal empire's wealth.

Explanation:

Taj Mahal was commissioned around 1632. At that time, the approximate cost of building Taj Mahal is around 32 Million Rupees. If that amount is converted to present day value, it cost the Mughal empire around 700 Billion Rupees or approximately 956 million US dollar.

Meanwhile , the annual  earning of the whole empire is only around half of that amount.

This led to various economics problems and force the Mughal empire to cut down their military's budget and make them weaker. Eventually, it led to the decline of the Mughal empire.

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