* KH (Potassium hydride) is a very reactive metal hydride. When it comes into contact with water, it undergoes a vigorous reaction, producing hydrogen gas (H₂) and potassium hydroxide (KOH):

KH + H₂O → KOH + H₂

Let's look at why the other options are incorrect:

* NH₃ (Ammonia) is a weak base and does not readily react with water to produce hydrogen gas.

* HF (Hydrogen fluoride) is a weak acid. While it reacts with water, it does not produce hydrogen gas. Instead, it forms hydronium ions (H₃O⁺) and fluoride ions (F⁻).

* CH₄ (Methane) is a very stable alkane and does not react with water under normal conditions.

* SF₆ (Sulfur hexafluoride) is an extremely unreactive compound and does not react with water.

* F₂ (Fluorine) is a very reactive element and will react with water, but the reaction is complex and does not primarily produce hydrogen gas. It forms a mixture of products, including oxygen, hydrogen fluoride, and ozone.

In summary, only KH reacts rapidly with water to produce hydrogen gas.