Propagation delay of a digital logic circuit
IMHO there are two anwsers:


"Infinite" - when B=0 &amp; D=1
14ns (in this idealized question).


The propagation delay of a signal is indeed the sum of all the delays in the longest path.
In the example, if A would also be an input to the 'OR' port, there might be two changes in the output, but the stable output is only reached after the longest path has settled.

The "infinite" case is not usually considered but I consider it for completeness.

In practice, propagation delays depend on the transition type (up/down), temperature, voltage and manufacturing process (those are the ones that are considered when designing integrated circuits).