________are sour in taste​.

Acids are compounds that give hydrogen ions, and because there are more hydrogen ions than hydroxide ions in an acidic solution, acidic substances have a sour flavour. This indicates that the more a food is seasoned, the more hydrogen ions are accessible to activate the receptors for sour flavours.

As we all know, the process of making curd involves fermentation. When lactobacillus bacteria are added to milk during the fermentation process, they transform the lactose sugar found in the milk into lactic acid. Lactic acid is what gives curd its sour flavour. And as for lemon juice, we are all aware that lemons contain citric acid, which has a sour taste. Lemon juice, therefore, tastes sour because of citric acid.

Citric acid, which has a sour flavour, is also present in orange juice. Orange juice, therefore, tastes sour because of citric acid.

Furthermore, vinegar is also referred to as acetic acid. The behaviour of acid is to taste sour. So vinegar also has a sour flavour.

Therefore, all of the alternatives include acid and have a sour flavour.

Therefore, Hence, the correct option is (D), All of the above.