Which of the following solutions will cause the blue litmus to turn red?

Vinegar

The correct answer is option 1. Vinegar.

Litmus paper is a commonly used pH indicator. It comes in two colors: blue and red. Blue litmus paper turns red in the presence of an acidic solution and remains blue or unchanged in neutral or basic solutions.

In the context of the provided options:

1. Vinegar:  Vinegar is an acidic solution containing acetic acid. When blue litmus paper is dipped into vinegar, the presence of acids in the solution causes the blue litmus paper to change color from blue to red.

2. Soap: Soap is typically alkaline or basic. When blue litmus paper is dipped into a basic solution, it will generally remain blue or might even turn a bit more blue, indicating that the solution is basic.

3. Milk of Magnesia:  Milk of magnesia is a basic or alkaline solution. When blue litmus paper is exposed to milk of magnesia, it will likely remain blue or show no significant change.

4. Calamine:  Calamine lotion is often considered neutral or slightly basic. Similar to milk of magnesia, it may have little to no effect on blue litmus paper, and the paper will likely remain blue.

In summary, vinegar is the most acidic solution among the options, causing the blue litmus paper to turn red.