Answer:
d. a change in interest rates, which changes investment.
Step-by-step explanation:
In economics, price of investment is interest rate. Therefore, a contractionary monetary policy will lead to higher interest rate and this will transmit to investment by reducing the level of investment as it now more expensive for investor to borrow loanable fund to invest.
On the other hand, an expansionary monetary policy will lead to lower interest rate and this will transmit to investment by increasing the level of investment as it now cheaper for investor to borrow loanable fund to invest.