Afirm measures its effectiveness in using the resources it has allocated to its marketing effort by use of which tool?

Marketing return on investment or in short ROI.
ROI calculates the sum returned on any investment made in relation to the cost of investment. Usually it is calculated by this method:
Benefit/ Return ÷ Investment cost = % or ratio

A firm may use a Return on Marketing Investment (ROMI) calculation to determine the effectiveness of its investment in marketing. This calculation can be performed a few ways, one of the most simple methods is to divide the increase in gross profit (minus the marketing cost), by the marketing cost. For example, if an ad campaign costs 5,000 and gross profit increases by 10,000, then the ROMI would be (10,000-5,000)/5,000)=100%. The return on Marketing Investment would be 100%. 

comment it always confuses me

(75years ,i believe is the correct answer.good luck