Let's analyze the proportion given:
[tex]\[
\frac{3}{4} = \frac{6}{8}
\][/tex]
In a proportion [tex]\(\frac{a}{b} = \frac{c}{d}\)[/tex], the terms [tex]\(a\)[/tex] and [tex]\(d\)[/tex] are called the "extremes", and the terms [tex]\(b\)[/tex] and [tex]\(c\)[/tex] are referred to as the "means".
For the proportion given:
[tex]\[
\frac{3}{4} = \frac{6}{8}
\][/tex]
The first term [tex]\(a\)[/tex] is 3, the second term [tex]\(b\)[/tex] is 4, the third term [tex]\(c\)[/tex] is 6, and the fourth term [tex]\(d\)[/tex] is 8. According to the definition of extremes, the extremes in this proportion are the first and the fourth terms.
Thus, the extremes of the proportion [tex]\(\frac{3}{4} = \frac{6}{8}\)[/tex] are:
[tex]\[
3 \text{ and } 8
\][/tex]
Therefore, the correct option is:
D. 3 and 8
