Main axis of diatomic molecule is Z. The orbitals p_x and p_y overlap to form:
A. π - molecular orbital
B. σ - molecular orbital
C. δ - molecular orbital
D. No bond is formed.

To solve this question, let's first understand the basic concepts of molecular orbitals and how orbitals overlap.

For a diatomic molecule, the main axis (often referred to as the internuclear axis) is the line connecting the nuclei of the two atoms. The type of molecular orbital formed depends on the orientation of the atomic orbitals that overlap.

1. **σ (sigma) Molecular Orbitals**:
- Formed by the end-to-end (head-on) overlap of atomic orbitals along the internuclear axis.
- Typically involves s-orbitals or p-orbitals (specifically \( p_z \)) that align along this axis.

2. **π (pi) Molecular Orbitals**:
- Formed by the side-to-side overlap of atomic orbitals.
- Involves \( p_x \) and \( p_y \) orbitals that are perpendicular to the internuclear axis.

3. **δ (delta) Molecular Orbitals**:
- Involves more complex overlapping and is less common in diatomic molecules. It generally requires d-orbitals.

4. **No Bond Formation**:
- This would imply that the orbitals do not overlap in a way that forms a molecular orbital.

Given that the orbitals \( p_x \) and \( p_y \) are perpendicular to each other and to the internuclear axis, they would overlap side-by-side, not along the axis. This type of overlap is characteristic of π molecular orbitals.

Therefore, when \( p_x \) and \( p_y \) orbitals overlap, they form **π-molecular orbitals**.

The correct answer is:

**A. π - molecular orbital**