First-order phase transitions are characterized by a sudden change in the properties of a substance at a specific temperature and pressure. These transitions involve a change in the state of matter, such as solid to liquid or liquid to gas.

In the given options, the first-order phase transitions can be identified as follows:

a) Vaporization of a liquid at its boiling point:
- When a liquid is heated to its boiling point, it undergoes a phase transition from liquid to gas.
- This transition is a first-order phase transition because it occurs at a specific temperature and pressure and involves a change in the state of matter.
- The transition is characterized by the sudden formation of vapor bubbles throughout the liquid, leading to the conversion of the entire liquid into a gas phase.

b) A liquid to gas transition close to its triple point:
- The triple point is the temperature and pressure at which all three phases of a substance (solid, liquid, and gas) coexist in equilibrium.
- When a liquid is heated and brought close to its triple point, it undergoes a first-order phase transition from liquid to gas.
- This transition is characterized by the sudden evaporation of the liquid into a gas phase, similar to the vaporization at the boiling point.

c) Ferromagnetic to paramagnetic:
- The transition from ferromagnetic to paramagnetic is not a first-order phase transition.
- It is a second-order phase transition because it does not involve a change in the state of matter but rather a change in the magnetic properties of a material.
- In a ferromagnetic material, the magnetic moments of the atoms align in a parallel manner, creating a strong magnetic field. In contrast, in a paramagnetic material, the magnetic moments are randomly oriented, resulting in a weaker magnetic response.

d) Superconducting to normal state:
- The transition from superconducting to the normal state is also not a first-order phase transition.
- It is a second-order phase transition because it does not involve a change in the state of matter but rather a change in the electrical conductivity of a material.
- In the superconducting state, the material exhibits zero electrical resistance, while in the normal state, it has finite resistance.

In summary, the first-order phase transitions among the given options are the vaporization of a liquid at its boiling point (option a) and a liquid to gas transition close to its triple point (option b).