It's nice to see you again. We  _____  each other since 2014.
A. won't see     B. don't see    C. haven't seen    D. didn't see

解析：
现在完成时，表示过去发生或已经完成的动作对现在造成的影响或结果，或从过去已经开始，持续到现在的动作或状态。根据It's nice to see you again. 可知已经见到了。根据since 2014，可知从过去已经开始，持续到现在的状态，应该用现在完成时。故选择C

It's nice to see you again. We  _____  each other since 2014.
A. won't see     B. don't see    C. haven't seen    D. didn't see

解析：
现在完成时，表示过去发生或已经完成的动作对现在造成的影响或结果，或从过去已经开始，持续到现在的动作或状态。根据It's nice to see you again. 可知已经见到了。根据since 2014，可知从过去已经开始，持续到现在的状态，应该用现在完成时。故选择C