Colonial boycotts of british goods before the revolutionary war were effective measures because they..
1) reduced the profits of the british merchants
2)lowered the prices of imported merchants
3) left the british troops short of supplies in the colonies
4) allowed the americans to start their own factories

Colonial boycotts of British goods before the Revolutionary war reduced the profits of the British merchants.

Explanation:

There were laws like Townsend Acts, passed in 1767 imposed taxes on tea, glass, paint, lead import. Colonists protested the same saying that these taxes violated their rights as British citizens. They thus started boycotting British goods i.e. stopped buying them. This boycott of British goods led to a series of boycotts which ultimately led to the American revolution.

Boycotts reduced the profits of British merchants. So, they even petitioned Parliament to revoke the Townshend Acts.  

the answer is:

c. economic output