First count the matching tuples.
For $\mathrm{cid}=1$, Joe matches two reservation tuples $:2$ tuples.
For $\mathrm{cid}=2$, Betty matches four reservation tuples $:4$ tuples.
For $\mathrm{cid}=4$, Harry matches one reservation tuple $:1$ tuple.
Therefore, the inner-join portion contains $2+4+1=7$ tuples.
Now consider dangling $\mathrm{Customer}$ tuples.
Sally, with $\mathrm{cid}=3$, has no reservation.
The full outer join preserves her $:+1$.
Now consider dangling $\mathrm{Reserves}$ tuples.
The reservation with $\mathrm{cid}=5$ has no corresponding customer.
The full outer join preserves it as well $:+1$
$\therefore 7+1+1=9$.
Hence, Answer : C
Note : Full Outer Join = matched tuples + dangling left tuples + dangling right tuples.
