The correct answer to the question is option 3, "Recumbent". In a recumbent fold, the axial plane, which is a theoretical plane that divides a fold into two symmetrical parts, is found to be virtually horizontal. A recumbent fold is characterized by the fact that the fold axis, which is an imaginary line running through the crest and trough of a fold, is almost parallel to the horizontal plane.

To better understand the other options, let`s briefly explain them:

1. Isoclinal fold: In an isoclinal fold, the axial plane is not necessarily horizontal. Instead, the axial plane is usually inclined at an angle.

2. Anticlinal fold: In an anticlinal fold, the axial plane is not usually horizontal. Instead, it is inclined and forms an arch-like structure with the fold axis pointing upwards.

4. Monoclinal fold: In a monocline, the axial plane is not horizontal either. It is inclined, and the fold axis is not parallel to the horizontal plane.

Therefore, the correct option is 3, "Recumbent," where the axial plane is found to be virtually horizontal.