Which option is an outcome of a project stated in quantifiable terms? a feeling of pride in a job well done
staff turnover rates
a 10 percent increase in efficiency of sales of a particular product line over one year
a letter of recommendation from a client who is happy with your company's work

C) a 10 percent increase in efficiency of sales of a particular product line over one year

The term "quantifiable" means that we can attach a number to it. Specifically, a number in which we can do math operations upon it. Something like telephone numbers or serial numbers don't really count as quantifiable entities. They are qualitative variables instead. So you have to be careful what numbers you run into. In this case, "10 percent increase in efficiency" is quantitative and fits what we're after.

A 10% increase corresponds to the multiplier 1.10; for instance, if you had sales of 40 million dollars one year, then a 10% increase is 1.10*40 = 44 million in sales. This is one example where we apply a math operation (specifically multiplication) on the value in question.