Parallel execution sequence
B, D incorrect:

The join is performed in parallel, there is only 1 serial operation on the picture, only one parallel->serial (P->S) transition, and that is when the query coordinator collects the results. 

A (, D) incorrect: 

Parallel SQL uses the producer-consumer model. One set reads the table, the other set builds/probes the hash table, they do not read seperate tables simultaneously.

C incorrect:

There is only 1 DFO (:TQ1), the order of execution can be easily recognized by the order of virtual tables or the table queues (:TQ1000x). This is not necessarily true for multiple DFOs, but it is inside 1 DFO. :TQ10000 comes before :TQ10001, but :TQ10000 does not necessarily come before :TQ20000. Read more about this:

https://jonathanlewis.wordpress.com/2015/12/21/parallel-plans-2/

So the first table to be read is CUSTOMER, not SALES.

E correct:

Well, this is how it works, but that is not a real explanation. Easier to prove the other options wrong.