The altitude at which best fuel mileage will occur for a given speed schedule, fixed M, LRC etc.
Sometimes operating cosiderably off optimum altitude can work out better if better winds are at a different altitude, hence the wind altitude trade table (Boeing) which looks at the increase or decrease in wind speed that is required to opperate off optimum altitude and break even. So the question above I suppoe assumes nil wind and a fixed M no? in which case yes 'b'
Interstingly enough the B738 fuel milage penilty for opperating 2k either side of optimum at LRC is the same, its only when you are 3k and below optimum that fuel milage penilty really starts to increase.
Best Rgds