Step 1: Understand unlike parallel forces.
Two parallel forces that point in opposite directions are called unlike forces. For such a pair, the single resultant force acts outside both of them, on the side of the bigger force. This is the picture we keep in mind.
Step 2: Use the moment rule.
To locate the resultant we balance turning effects, called moments. A moment is force times its distance. For the resultant point, the moments of the two given forces must balance: $F_1 d_1=F_2 d_2$.
Step 3: List the data.
The forces are $F_1=2\,\text{N}$ and $F_2=16\,\text{N}$, sitting at the ends of a rod $21\,\text{cm}$ long. Let the resultant be at distance $x$ from the larger $16\,\text{N}$ force. Then it is $21+x$ from the smaller force, since it lies outside on the big force side.
Step 4: Write the moment balance.
Set the moment of the big force equal to the moment of the small force about the resultant point. \[ 16x=2\,(21+x) \]
Step 5: Simplify.
Open the right side. \[ 16x=42+2x \] Bring the $x$ terms together: $16x-2x=42$, so $14x=42$.
Step 6: Solve for $x$.
Divide both sides by $14$. \[ x=\frac{42}{14}=3\,\text{cm} \] So the resultant acts $3\,\text{cm}$ from the greater force, which is option 2. \[ \boxed{3\ \text{cm}} \]