Electric flux is a measure of the electric field passing through a given area. It is defined as the product of the electric field strength and the area over which it is applied. The dimensional formula of electric flux can be determined by analyzing the units of the variables involved in its calculation.

Electric field strength (E) is defined as the force per unit charge experienced by a test charge placed in the electric field. The formula for electric field strength is given by:

E = F/q

where F is the force experienced by the test charge and q is the magnitude of the test charge.

The SI unit of electric field strength is newtons per coulomb (N/C), which can be expressed as kg·m·s^(-2)·C^(-1). Therefore, the dimension of electric field strength is:

[M^1L^1T^(-2)A^(-1)]

The area (A) over which the electric field is applied is measured in square meters (m^2), which has the dimensions of:

[L^2]

Therefore, the dimensional formula of electric flux is given by:

[M^1L^1T^(-2)A^(-1)] * [L^2] = [M^1L^3T^(-2)A^(-1)]

Hence, the correct answer is option B - [M^1L^3T^(-3)A^(-1)].

In summary, the dimensional formula of electric flux is obtained by multiplying the dimensions of electric field strength and area, resulting in the formula [M^1L^3T^(-3)A^(-1)].