tech a says that a slightly lean mixture offers good fuel economy and low exhaust emissions. Tech b says that a mixture that is too rich fouls spark plugs and causes incomplete burning. Who is correct

Both Techs A and B are correct. The correct option is C.
Explanation:
Exhaust emissions are gases that are expelled from vehicle engines and they include carbon monoxide, unburned fuel, nitrogen oxides and particulate matter such as mercury. These emissions can cause air pollution and green house effects which leads to climate changes. Low exhaust emissions can be achieved through the use of lean mixtures which has high air- fuel ratio and less emission. Therefore, Tech A is correct.
Rich mixtures are mixtures that are of low air- fuel ratios and it's burning in a diesel engine increases particulate matter emission, fouls spark plugs, causes incomplete burning. Therefore Tech B is equally correct. Hope this helps, thanks.