Step 1: Recall the character of the nuclear force.
The nuclear force between nucleons is very short ranged and changes character with separation.
Step 2: Note the repulsive core.
At very small separations, roughly below about $0.8\,fm$, the force becomes strongly repulsive, which keeps nucleons from collapsing together.
Step 3: Note the attractive region.
For separations roughly between $0.8\,fm$ and a few femtometres, the force is attractive, which is what binds the nucleus.
Step 4: Note the long-range fall-off.
Beyond a few femtometres the nuclear force becomes negligible.
Step 5: Test the options.
$0.12\,fm$, $10^{-3}\,fm$ and $0.3\,fm$ all lie in the repulsive core (below $0.8\,fm$), so they cannot give attraction. Only $1.1\,fm$ lies in the attractive window.
Step 6: Conclude.
For the force to be attractive the separation must be about $1.1\,fm$, which is option (3).
\[ \boxed{1.1\,fm} \]