The curve is bell shaped, the distribution of data is equal on both sides , that is from mid point called mean.
50% of Data lies on both side of mean.
So,Equation 1.→ Mean - 2 × standard deviation=3.5
or,Equation 2. →Mean + 2 × standard deviation=4.5
x -2 a= 3.5
4 -2 a=3.5
4-3.5=2 a
2 a=0.5

Using equation (2)
→ 4 +2 a=4.5
2 a=4.5-4
2 a=0.5

So, Standard Deviation = 0.25
→Variance =(Standard deviation)²=a²=(0.25)²=0.0625
Option (A)→0.0625