Final answer:
An object that fills one-quarter of a microscope's field with a diameter of 0.12mm would have a width of 0.03mm (option a).
Step-by-step explanation:
If the diameter of a microscope's field is 0.12mm, an object that fills one-quarter of the field has a width of 0.03mm. This can be determined by dividing the total diameter of the field by four, since one-quarter of the circle will have a diameter that is one-quarter the length of the entire diameter.
To calculate:
Object width = (Field diameter) / 4
Object width = 0.12mm / 4
Object width = 0.03mm
Therefore, the correct answer is option a. 0.03 mm.