Ashish leaves his house at 20 minutes to seven, which is 6:40 a.m.
It takes him 25 minutes to reach Kunal's house:
\(⇒\) 6:40 a.m. + 25 minutes = 7:05 a.m.
They finish breakfast in 15 minutes:
\(⇒\) 7:05 a.m. + 15 minutes = 7:20 a.m.
Thus, they leave Kunal's house at 7:20 a.m. to reach their office.
So, the correct option is (B): 7.20 a.m.