The plastic modulus of a section is a measure of its ability to resist plastic deformation. It is defined as the product of the shape factor and the plastic moment capacity of the section. The shape factor relates the plastic modulus to the section's geometry.

Given:
Plastic modulus = 4.8 x 10^-4 m^3
Shape factor = 1.2
Plastic moment capacity = 120 kN.m

To find the yield stress of the material, we can use the formula for plastic modulus:

Plastic modulus = Shape factor x Plastic moment capacity

4.8 x 10^-4 m^3 = 1.2 x 120 kN.m

Simplifying the equation, we get:

4.8 x 10^-4 = 144

Now, let's convert the plastic modulus to square meters:

Plastic modulus = 4.8 x 10^-4 m^3 = 4.8 x 10^-4 x 10^6 mm^3 = 480 mm^3

Since the plastic modulus is equal to the shape factor multiplied by the plastic moment capacity, we can rearrange the equation to solve for the shape factor:

Shape factor = Plastic modulus / Plastic moment capacity

Shape factor = 480 mm^3 / 120 kN.m

Shape factor = 4 mm^3 / kN.m

Now, let's convert the shape factor to square meters:

Shape factor = 4 mm^3 / kN.m = 4 x 10^-6 m^3 / 10^3 N.m = 4 x 10^-9 m^3 / N.m

We know that the shape factor is given as 1.2, so we can solve for the plastic moment capacity:

1.2 = 4 x 10^-9 / N.m

N.m = 4 x 10^-9 / 1.2

N.m = 3.333 x 10^-9

Now, let's convert the plastic moment capacity to kilonewton-meters:

Plastic moment capacity = 3.333 x 10^-9 x 10^-3 = 3.333 x 10^-12 kN.m

Finally, let's find the yield stress of the material using the plastic moment capacity:

Yield stress = Plastic moment capacity / Plastic modulus

Yield stress = 3.333 x 10^-12 kN.m / 4.8 x 10^-4 m^3

Yield stress = 6.944 x 10^-9 kN/m^2

Converting the yield stress to megapascals (MPa):

Yield stress = 6.944 x 10^-9 kN/m^2 = 6.944 x 10^-6 MPa

Therefore, the yield stress of the material is approximately 6.944 MPa. None of the given options match this value, so the correct answer is not provided.