Which of the following is a base?

Window cleaner

The correct answer is option 4. Window cleaner.

Window cleaner is often a basic solution, and it can contain ammonia or other alkaline substances. The presence of ammonia or other bases in window cleaners helps in breaking down and removing dirt, grease, and grime from glass surfaces. Ammonia, in particular, is an alkaline compound and can act as a base.

The other options mentioned:

1. Curd: Curd is not typically considered a base. It is a dairy product and is slightly acidic in nature.

2. Vinegar:  Vinegar is an acidic solution. It contains acetic acid, which imparts a sour taste and acidic properties.

3. Amla:  Amla (Indian Gooseberry) is not a base. It contains ascorbic acid (vitamin C) and is generally considered acidic.

In summary, window cleaner is more likely to be a basic solution due to the presence of alkaline compounds like ammonia, while curd, vinegar, and amla are not bases; curd is slightly acidic, and vinegar and amla are acidic.