At what co2 concentration was stomata aperture width at its greatest? lowest ?

Stomata aperture width is at its greatest when there is low concentration of CO2 in the atmosphere.

As the air's CO2 content rises, many plants reduce their stomatal apertures. As a result, plants growing in CO2-enriched air typically reduce the density of stomates on their leaf surfaces and as a consequence, exhibit reduced rates of transpirational water loss, smaller productivity losses attributable to the indiscriminate uptake of aerial pollutants, and increased water-use efficiency.  

nucleus

would be most reasonable answer tbh