Answer:
B，8.8cm
Explanation:
I don't know if you have learned about the relations between the three sides of a 30,60,90 right-angled triangle. It shows one angle is 29, we will estimate it for 30.
Relations： 30 angled face side DF is 1, 60 face side EF is √3=1.732， 90 face side DE is 2
And we knew EF=7.7
So, 1.732/7.7=2:DE
1.732DE=15.4
DE=8.89
Sorry, I hope someone can help you in a simpler way.