Write one source from which the government of Bhutan
borrow to fill the gap between planned expenditure and the
estimated revenue?

External borrowing from India

Explanation:

The Government of Bhutan has planned to spend more than it collects from various revenue streams. Taxes form the biggest source of revenue for any government. The gap between a country's planned expenditure and the estimated revenue is the fiscal deficit.

The government of Bhutan finances its budget deficits through external borrowing. India is Bhutan's biggest trading partner and financier.  

answer; lags, push the econ below the natural level of output;