If referring to the question: Kathy was making liquid growth medium for plants. The pH of the original solution was 8. The optimal pH for plant growth is 6. What was the best compound to add to make it pH=6?

A) KOH

B) HCL

C) Water

D) No answer provided

Then the correct answer would be option B, to add HCL, known as Hydrochloric acid to lower the pH to 6.

In science, pH is a scale used to measure the acidity of a solution or its basicity. The scale ranges from 0 to 14. A pH of 7 would mean that the given solution is neutral, lower than 7 equates to an acidic solution, while anything above 7 becomes increasingly basic.

Option A is to add KOH, or potassium hydroxide to the original solution whose starting pH is 8. KOH is a solution whose pH is above 8, a strong base, and is often used to increase the pH of a solution. In the question it states what the desired pH is 6, therefore we want to lower, not increase the pH. This makes option A incorrect.

Water is a neutral solution. Even though it is neutral, it is not used to neutralize basic or acidic solutions. Acid or bases may be added to water to create very diluted solutions that are nearly neutral despite containing a strong base or acid, but this is a different process than neutralization and will not give the desired pH. Option C is also wrong.

Option B is correct because HCL, known as hydrochloric acid is a strong acid, commonly used to lower the pH of a solution by increasing the concentration of hydrogen ions. Since we start with a pH of 8 and wish to achieve a pH of 6, HCL is the correct component to add to the solution. Due to option B being correct, option D that states that no answer is provided, is incorrect.