The correct option is B Earthen pot
Metals react with acid to produce metal salt and hydrogen gas. Buttermilk contains lactic acid which can react with metal and affect our health badly. So, buttermilk cannot be stored in aluminium, magnesium or iron containers. It is advisable to not store food materials into plastic bottles as they are also made up of many harmful chemicals. Therefore, earthen pot is best suitable to store buttermilk.