Parallel Fold:
A parallel fold is a type of fold found in rock layers or geological formations. It is characterized by its distinctive shape and structure, where the fold appears as a series of parallel ridges or undulations.

Characteristics of Parallel Fold:
To understand why option A is the correct answer, let's examine the characteristics of a parallel fold:

1. Orthogonal Thickness: In a parallel fold, the orthogonal thickness is the same all around the fold. This means that the fold maintains a consistent thickness perpendicular to its axial plane.

2. Axial Plane: The axial plane of a fold is an imaginary plane that divides the fold into two symmetrical halves. In a parallel fold, the thickness remains the same parallel to this axial plane.

3. Outer and Inner Arcs: A parallel fold is defined by its outer and inner arcs, which refer to the curves formed by the fold's layers on the outside and inside of the bend. In a parallel fold, the outer and inner arcs have the same curvature.

4. Dip Isogons: Dip isogons are lines that connect points of equal dip along the fold. In a parallel fold, these dip isogons are parallel to each other, indicating a consistent dip angle along the fold.

Explanation of the Correct Answer:
Now let's analyze the given options and see why option A is the correct answer:

a) The orthogonal thickness is same all around the fold:
- This statement accurately describes the characteristics of a parallel fold. The orthogonal thickness remains consisent all around the fold, making option A the correct answer.

b) The thickness is same parallel to the axial plane of the fold:
- This statement is incorrect. In a parallel fold, the thickness remains the same perpendicular to the axial plane, not parallel to it.

c) The outer and inner arcs have same curvature:
- This statement is partially correct. In a parallel fold, the outer and inner arcs do have the same curvature, but this is not the defining characteristic of a parallel fold.

d) The dip isogons are parallel to each other:
- This statement is incorrect. While dip isogons may be parallel in some types of folds, it is not a defining characteristic of a parallel fold.

In conclusion, option A accurately describes the characteristics of a parallel fold, making it the correct answer.