Final answer:
An object that fills one-quarter of a microscope's field with a diameter of 0.12mm would be 0.03mm wide, corresponding to answer choice a.
Step-by-step explanation:
If the diameter of a microscope's field is 0.12mm, then to find out how wide an object that fills one-quarter of the field is, you can simply divide the diameter by 4 (since one-quarter is the same as dividing by 4). So, the calculation would be 0.12mm / 4 = 0.03mm. Therefore, an object that fills one-quarter of the field would be 0.03mm wide which corresponds to answer choice a. 0.03 mm.