My buddy and I have just finished a dive to 15 m/50 ft for 60 minutes. We want to return to the same site and depth and stay another 60 minutes. We can to see about how long we have to remain at the surface to have enough no stop time
A. Wait and check our no stop times on our computers periodically
B. Use the dive computer plan mode, if our computers have them
C. Use a dive planning app
D. Check the RDP Table or eRDPml

All of the options applies

Explanation:

Dive computers can be used to check diver's no stop times which is the time spent at a given depth, therefore option A is applicable to calculate how long they have to remain at the surface to have enough no stop time

dive computers are equipped planning mode called simulation to calculate remaining surface interval with the depth of the dive. therefore option B is applicable to calculate how long they have to remain at the surface to have enough no stop time.

Also different dive planning apps could be used, as well as Recreational Dive Planner (RDP) Table