Among the following surface hardening processes, steel is heated to the lowest temperature in:

The correct option is nitriding.

To understand why nitriding requires the lowest heating temperature among the given surface hardening processes, let us compare the typical processing temperatures for each method:
1. Carburizing: This process involves diffusing carbon into the surface of low-carbon steel. It is performed in the austenitic region, typically at temperatures ranging from 900 °C to 950 °C.
2. Cyaniding: This process introduces both carbon and nitrogen simultaneously into the steel surface using a liquid salt bath. It is generally carried out at temperatures between 800 °C and 900 °C.
3. Carbonitriding: Similar to cyaniding but usually gas-based, this process also diffuses both carbon and nitrogen. It is performed at temperatures of about 760 °C to 870 °C.
4. Nitriding: This process diffuses nascent nitrogen into the steel surface to form hard nitrides. Unlike the other methods, it is a sub-critical heat treatment process and is carried out in the ferritic region, typically at temperatures between 480 °C and 590 °C.

Comparing these temperature ranges:
$\text{Nitriding (480 °C - 590 °C)}$ is significantly lower than $\text{Carbonitriding (760 °C - 870 °C)}$, $\text{Cyaniding (800 °C - 900 °C)}$, and $\text{Carburizing (900 °C - 950 °C)}$.
Therefore, steel is heated to the lowest temperature in the nitriding process.