In which one of the following folds is the axial plane found to be virtually horizontal?

A recumbent fold is characterized by an axial plane that is essentially or virtually horizontal [1]. This type of fold occurs when the crest of a fold is pushed so far that the axial surface dips at a very low angle, typically between 0° and 10° relative to the horizontal [2]. In such structures, the limbs are also nearly horizontal, and one limb is typically inverted [1]. In contrast, isoclinal folds have parallel limbs but their axial planes can be at any orientation [1]. Anticlinal folds are defined by their convex-upward shape rather than a specific axial plane orientation [3], and monoclinal folds represent a step-like transition between horizontal layers. Therefore, the recumbent fold is the specific classification where the axial plane is virtually horizontal [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 10: Types of Mountains > Types of Folds > p. 134
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Types of Mountain > p. 22
3. [3] https://www.britannica.com/science/fold