Which one has a pyramidal shape?

The correct option is PCl₃.

To determine why PCl₃ (Phosphorus trichloride) has a pyramidal shape, we can analyze the valence shell electron pair repulsion (VSEPR) theory for each of the given species:

1. Phosphorus trichloride (PCl₃):
- The central atom is Phosphorus (P), which belongs to Group 15 and has 5 valence electrons.
- It forms three single covalent bonds with three Chlorine (Cl) atoms, using 3 of its valence electrons.
- The remaining 2 valence electrons form 1 lone pair on the phosphorus atom.
- According to VSEPR theory, a steric number of 4 (3 bonding pairs + 1 lone pair) corresponds to a tetrahedral electron-pair geometry.
- Due to the presence of the one lone pair, the molecular geometry is distorted from tetrahedral to trigonal pyramidal (or simply pyramidal).

2. Sulfur trioxide (SO₃):
- The central atom is Sulfur (S), which has 6 valence electrons and forms double bonds with three Oxygen (O) atoms.
- It has 3 bonding regions and 0 lone pairs on the sulfur atom, giving it a trigonal planar shape.

3. Carbonate ion (CO₃^2-):
- The central atom is Carbon (C), which has 4 valence electrons.
- It forms bonds with three Oxygen atoms (with a net 2- charge), resulting in 3 bonding regions and 0 lone pairs on the carbon atom.
- This gives it a trigonal planar shape.

4. Nitrate ion (NO₃^-):
- The central atom is Nitrogen (N), which has 5 valence electrons.
- It forms bonds with three Oxygen atoms (with a net 1- charge), resulting in 3 bonding regions and 0 lone pairs on the nitrogen atom.
- This also results in a trigonal planar shape.

Therefore, among the given choices, only PCl₃ has a pyramidal shape.