Option B:
It is one-half the area of a square of side length 4 units.
Solution:
Base of the triangle = 4 units
Height of the triangle = 4 units
Area of the triangle = 

Area of the triangle = 8 square units
Option A: It is one-half the area of a rectangle of length 4 units and width 2 units.
Area of the rectangle = 4 × 2 = 8 square units
Half of rectangle =
square units
It is not true because area of the triangle is 8 square units.
Option B: It is one-half the area of a square of side length 4 units.
Area of the square = 4 × 4 = 16 square units
Half of square =
square units
It is true because area of the triangle is 8 square units.
Option C: It is twice the area of a rectangle of length 4 units and width 2 units.
Area of the rectangle = 4 × 2 = 8 square units
Twice of rectangle = 2 × 8 = 16 square units
It is not true because area of the triangle is 8 square units.
Option D: It is twice the area of a square of side length 4 units.
Area of the square = 4 × 4 = 16 square units
Twice of square = 2 × 16 = 32 square units
It is not true because area of the triangle is 8 square units.
Hence Option B is the correct answer.
It is one-half the area of a square of side length 4 units.