Except there is a conflation with those definitions that I think was intentional so as to simplify.
A) is certainly centralised.
B) is certainly decentralised.
C) is also decentralised but just a different organisation.

B) is a Hierarchical decentralisation - commonly used in military communications and c) is a Mesh decentralisation commonly used in ad-hoc wireless.

So where does distributed come in to it?
Distributed is used to describe how work or computation is partitioned within a network. Things like BOINC are distributed and it is quite possible to have decentralised and distributed as well as centralised and distributed (where BOINC is the latter).

In a nutshell. Decentralised != Distributed in the same way a Road != a Car

As an example, lets take a mining pool. So in a mining pool,  the mining *work* is distributed to the participants but the network architecture is B) hierarchical decentralisation (around the work coordinator/ provider). All communications within the pool are worker to and from the admin and the admin communicates the results to the wider network and relays any benefits back to the workers. In the early days of Bitcoin, each client was a miner and was its own admin where block mining was distributed rather than distributed mining of a block. It was distributed computation within a mesh network.

The issue we have at present is there is that full node clients are mesh networked to create their view of the block chain but all mining is Hierarchically decentralised and as the number of full nodes (required for acceptance of the miners' blocks) decreases, miners will start up their own botnets to propagate their blocks faster. Therefore they will own the means of production and the means of distribution; centralisation will be complete and you will have large mining corporations sitting in Google Data-centers with everyone using webs wallets.

Because mining is no longer "distributed" as originally envisioned, a decentralised network is counter-productive to mining efficiency. In the old days, a client was a miner and thus all mining was distributed and decentralised and until that is the case again, we will see more and more centralisation unless its already too late.