How do clover mites generally present themselves in terms of visibility?

Clover mites typically measure around 0.75 mm in size, which is about the thickness of a pinhead, making them quite small. They are often described as being nearly invisible to the naked eye due to their diminutive size. These mites can sometimes be seen when they form large groups, but when observed individually, they are challenging to detect.

Their appearance generally features a bright red or greenish color, which can be more noticeable when they are present in larger numbers or when they are moving across surfaces. However, their size is the primary reason they are considered difficult to see, particularly to untrained observers or those not specifically looking for them.

In contrast, other characteristics like being large and easily visible or crawling quickly can apply to different pests or insects and not clover mites specifically. Thus, option D accurately captures the characteristics of clover mites regarding their visibility.