Fresh milk has a pH of 6. To delay its curdling, a chemical substance is added to it, which is :

Curdling of milk is caused by the formation of lactic acid, which lowers the pH of the milk and makes it acidic. To delay this process, a mild, non-corrosive basic substance must be added to neutralize the acid as it forms.

• Option (d) is correct because baking soda (sodium hydrogen carbonate, $NaHCO_3$) is a mild, edible basic substance. Adding it to fresh milk slightly increases its pH (making it more alkaline), thereby delaying the curdling process as it takes longer for the lactic acid to lower the pH.
• Option (a) is incorrect because sodium carbonate ($Na_2CO_3$) is a stronger base and is not typically used as a direct food additive for preserving fresh milk.
• Option (b) is incorrect because baking powder is a mixture of baking soda and a mild edible acid (like tartaric acid), which would not be as effective in raising the pH of milk.
• Option (c) is incorrect because sodium hydroxide ($NaOH$) is a strong, highly corrosive alkali that is unsafe for consumption, as highlighted in the warning in the text.