The pipe joint mostly used for pipes carrying water at low pressures is

For pipes carrying water at low pressures, the most commonly used pipe joint is the Socket joint.

A Socket joint is simple to make and provides a tight connection suitable for low-pressure applications.

- Option A: Socket joint is widely used in plumbing for water supply lines at low pressure.
- Option B: Nipple joint is typically a short piece of pipe used to connect two fittings but is not a joint type by itself.
- Option C: Union joint is used where frequent dismantling is required, more suitable for maintenance purposes.
- Option D: Spigot and socket joint is similar to the socket joint but generally used for higher pressure or different pipe types.

Hence, the correct answer is Option A: Socket joint.