Explanation:

Quenching is the process of rapidly cooling a heated material in order to obtain specific properties, such as increased hardness. The choice of quenching media is important as it can significantly affect the final properties of the material. In this case, we are considering the quenching media for steel undergoing heat treatment.

The correct sequence of the given quenching media in order of increasing hardness of steel is as follows:

1. Oil: Oil is a relatively slow quenching medium compared to other options. It provides a moderate cooling rate, which results in a lower hardness compared to faster quenching media. However, it also offers a more uniform cooling process, reducing the risk of distortion or cracking.

2. Water: Water is a faster quenching medium compared to oil. It provides a rapid cooling rate, which leads to a higher hardness in the steel. Water quenching can result in a more brittle structure, which may increase the risk of cracking or distortion. Therefore, it is important to carefully control the cooling process to minimize these risks.

3. WaterNaOH: WaterNaOH refers to water mixed with sodium hydroxide (NaOH). This mixture is often used as a quenching medium to increase the cooling rate compared to plain water. The addition of NaOH enhances the cooling process, resulting in a higher hardness of the steel.

4. Brine: Brine is a solution of water and salt, usually sodium chloride (NaCl). It is one of the fastest quenching media available. The salt in the brine solution further reduces the cooling rate, leading to an even higher hardness in the steel compared to water or oil. However, the use of brine can increase the risk of cracking or distortion due to the rapid cooling rate.

Therefore, the correct sequence of these quenching media in order of increasing hardness of steel undergoing heat treatment is 1) Oil, 2) Water, 3) WaterNaOH, and 4) Brine.