If the central bank wants to the economy to grow, it can decrease
a. spending
b. interest rates
c. tax levels

b. interest rates

Explanation:

Interest rates are known as the sum of money which a borrower pays to a lender for borrowing money from him.

The Central Bank helps the economy to grow by lending money to commercial banks at a low interest rate. Furthermore, people also borrow more money from commercial banks because they also give out loan to them at a low interest rate.  Thus, this leads to increase in the amount of money in the economy which makes it to grow.