What is your current understanding of the question? It helps to share your thought process so other can provide a better answer.

Good idea Kate.

In order to size any storage array for an Exchange 2010 environment we require the following-

     1)Total # of users

     2)User I/O profile or # of messages sent/received per day

     3)Mailbox size limit

     4)Read/Write ratio

     5)Average message size

     6)Deleted items retention policy

     7)User concurrency requirements

     8)Log protection buffer

     9)Is replication requested ?

     10)RPO/RTO

In the aforementioned question, 5 out of the 10 metrics are provided which is okay since we need to calculate the minimum net capacity accounting 20% growth factor

Database size = (Total # of users)*(mailbox size on disk)*(database overhead growth factor)

Total # of users = 500

Mailbox size on disk = mailbox size limit + whitespace + dumpster    

    mailbox size limit = 1GB = 1024MB

    whitespace = (# of messages sent/received per day)*(Average message size)

                     = (125)*(65/1024)

                      ~ 8MB

    dumpster = ((# of messages sent/received per day)*(Average message size)*(Deleted items retention policy)) + (mailbox                        size limit*0.012) + (mailbox size limit*0.03)

                   = 111 + 12.29 + 30.72

                   = 154MB

Mailbox size on disk = mailbox size limit + whitespace + dumpster = 1024 + 8 + 154 = 1186MB

Database overhead growth factor = 1.2 (20% growth)

Database size = 500*1186*1.2

                      = 711600MB

Now, as per the question "Content indexing will be enabled". Thus, another 10% of capacity is to be added.

Database size = 711600*1.1

                      = 782760MB ~ 765GB

The option, in the question, closest to the attained output is 800GB.

Kindly correct me if I've got it wrong.

Thanks in advance !!!

Thank you sir