Considering x-axis as the internuclear axis, which out of the following will not form a sigma bond
and why ? `(a) s and 1s (b) 1s and 2p_(x) (c) 2p_(y) and 2p_(y)` (d) 1s and 2s`.

Explanation

To determine which of the given pairs of atomic orbitals will not form a sigma bond, we need to analyze the types of overlaps that occur between the orbitals when the x-axis is considered as the internuclear axis.

### Step-by-Step Solution:

1. **Understanding Sigma Bonds**:
- Sigma bonds are formed by the head-on (axial) overlap of atomic orbitals. This can occur between two s orbitals, an s and a p orbital, or two p orbitals if they are aligned along the internuclear axis.

2. **Analyzing Each Option**:
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