If two waves traveling in the same direction overlap and thus interfere, which best describes the interference?

Fully constructive intereference

All of the above. Waves traveling in opposite directions have to at some point to completely overlap and will have either full constructive or full destructive interference depending on if they are in phase or out of phase. Where they fully overlap will depend on phase difference. Waves traveling in the same direction may never fully overlap or they may always overlap, depending on phase. In the case when they fully overlap the interference can be fully constructive or fully destructive if phase difference is 0 or Pi radians (180 degrees). For phase difference between 0 and 2 Pi all possible interferences are possible.