$D = 5$ (and is not a funtion of $n$ here) and goes $1 - 2 -
4 - 5 - 6$.
What is the minimum number of processors that are needed to achieve
It clearly cannot be achieved with one processor, ...
The work $W(n)$ is the total number of nodes in your computation
graph and the span $D(n)$ is the number of nodes on the longest path
of that graph. $T_P(n)$ is the runtime of the algorithm
using $P$ ...