Javelle water is a

Answer: Option B

Javelle water, also known as sodium hypochlorite solution, is commonly used as a bleaching agent. It is effective in removing stains and whitening fabrics.

Reasoning:

Option A: blueing agent - This is incorrect because blueing agents are used to counteract yellowing in white fabrics, not for bleaching.
Option B: bleaching agent - This is correct because Javelle water is used to bleach and remove stains from fabrics.
Option C: finish - This is incorrect because a finish refers to treatments applied to fabrics to improve their appearance, feel, or performance, not to bleaching.
Option D: starch - This is incorrect because starch is used to stiffen fabrics, not to bleach them.

Therefore, the correct answer is Option B: bleaching agent, as Javelle water is primarily used as a bleaching agent in fabric care.