Awave front has the form of a
a. sine wave.
b. straight line.
c. circle.
d. surface of a sphere.

The rigth answer is D.

The wavefront or wavefront is an equal phase surface of a wave, i.e., these points have the same travel time from the source. The concept is used to describe the propagation of waves such as sound or electromagnetic radiation (light, radio waves, etc.). In a homogeneous and isotropic medium, in which the waves propagate without deformation, two types of particular waves are distinguished according to whether the wavefronts are spheres (spherical wave) or planes (plane wave).

currents is the correct answer.