The question asks to calculate the radius of a circle, where the diameter (d) is given as 6.8 mm.

The formula that relates the diameter ($d$) and the radius ($r$) of a circle is:

$r = \frac{d}{2}$

Substituting the value of the diameter:

$r = \frac{6.8 \text{ mm}}{2} = 3.4 \text{ mm}$

So, the correct answer is:

3.4 mm

Would you like more details or have any further questions?

Here are some related questions to expand on this topic:

1. How do you calculate the circumference of a circle using the radius?
2. What is the area of a circle if the radius is 3.4 mm?
3. How would you convert this radius from millimeters to centimeters?
4. If the diameter was given in centimeters, how would the calculation change?
5. How does the relationship between the radius and diameter help in understanding other circular properties?

Tip: Remember, the radius is always half of the diameter for any circle!