Line CD is the one shown below in yellow.
Let us compare it to each of the options.
AD
As we can see, segments AD and CD are different. So AD is not the correct name for the CD line.
CE
As we can see, segments CE and CD are different. So CE is not the correct name for the CD line.
DC
As we can see, segments DC and CD are equal. So DC is the correct name for the CD line.
DE
As we can see, segments DE and CD are different. So DE is not the correct name for the CD line.
In summary, we have:
AD: No
CE: No
DC: Yes
DE: No