D-glucose and D-galactose are C4-epimers.

Explanation:
To understand why D-glucose and D-galactose are C4-epimers, let's first define what epimers are. Epimers are a type of stereoisomers that differ in the configuration at only one chiral carbon.

Chiral Carbon: A chiral carbon is a carbon atom that is bonded to four different groups or atoms.

D-glucose:
D-glucose is an aldohexose, which means it is a 6-carbon sugar with an aldehyde group (-CHO) at carbon 1. It is also known as the "blood sugar" and is the most abundant monosaccharide in nature. D-glucose has the following structure:

H
|
HO-CH2OH - C - OH
|
H
|
HO-CH2OH - C - H
|
H
|
HO-CH2OH - C - H
|
H
|
HO-CH2OH - C - H
|
H
|
HO-CH2OH - C - H
|
H

D-galactose:
D-galactose is an aldohexose and an epimer of D-glucose. It is commonly found in lactose, a disaccharide composed of D-glucose and D-galactose. D-galactose has the following structure:

H
|
HO-CH2OH - C - OH
|
H
|
HO-CH2OH - C - H
|
OH
|
HO-CH2OH - C - H
|
H
|
HO-CH2OH - C - H
|
H
|
HO-CH2OH - C - H
|
H

C4-epimers:
C4-epimers are sugar molecules that differ in the configuration at carbon 4. In the case of D-glucose and D-galactose, the only difference is the configuration at carbon 4. In D-glucose, carbon 4 has an -OH group, while in D-galactose, carbon 4 has an -H group.

Therefore, D-glucose and D-galactose are classified as C4-epimers because they differ in the configuration at carbon 4.