The (internal) operation of different memory modules may overlap in time
Whenever operations can be overlapped, we can do them simultaneously. Whenever operations can be interleaved, we can use the pipelining concept. This is elementary stuff.
Now, read this line further.
The (internal) operation of different memory modules may overlap in time, but only one request can be on the bus at any time.
This is basically just interleaving. We can use the pipelining concepts here.
Now, see carefully what the question asks for.
The maximum number of stores (of one word each) that can be initiated in 1 millisecond is
Each initiation takes $100ns$. We can't overlap initiations, so they must be done separately.
In 1ms, we can perform $\frac{1ms}{100ns}=10^4=10000$ initiations. Answer.
If the question asked the number of stores that can be completed in $1ms$
We could write it as
$(100ns+500ns)+x(100ns)=1ms$
ie, the first job would take 100ns + 500ns time. The rest of the jobs would take just 100ns time, because only initiation is to be kept separate, internal operations can be overlapped. This equation is pretty much identical to what we use in pipelining.
Upon solving, $x=9994$
Since $x$ denotes "rest of the jobs", we need to add $1$ to $x$ to obtain total jobs.
So, total stores completed would be $9994+1=9995$