Oxalic acid is found in:
(A) Spinach
(B) Amla
(C) Grapes
(D) Unripe mangoes

Hint :Oxalic acid having a chemical formula of ${{C}_{2}}{{H}_{2}}{{O}_{4}}$ . It is an organic compound and forms a colourless solution in water. It acts as a reducing agent and occurs naturally in many foods. It can be dangerous for skin when used in excessive amounts.

Complete Step By Step Answer:
Let us discuss the option given in the question,
Spinach is rich in iron which plays a very important role in the functioning of red blood cells for the transportation of oxygen throughout the body. Spinach is rich in oxalic acid around $100-900mg$ per serving. It helps in preventing lots of calcium absorbed in the body. Spinach is an excellent source of vitamin A, vitamin K, manganese, vitamin B12. Vitamin K helps in making bones stronger and healthier. The presence of vitamin K in vegetables is very rare. Therefore option A is correct.
Amla is rich in vitamin C, Calcium and iron. It helps in improving the immunity of a human being. It is a very good medicine for heart diseases. It helps in thicken and strengthen the arteries of the human body. It is also good for skin and hair. It acts as a rich source of fibre. Therefore option B is incorrect.
Grapes are rich in vitamin K and vitamin C. It acts as a powerful antioxidant which is used for connecting tissue health. It also helps in preventing chronic diseases. Grapes do not contain oxalic acid. Therefore option C is also correct.
Unripe mangoes contain a very little amount of oxalic acid. It is more present in ripe mangoes. Therefore option D is also incorrect.

Note :
In this question, we concluded that spinach contains the highest amount of oxalic acid. When frozen spinach is grilled, it will show no opportunity for soluble oxalates to leach out during cooking. When soluble oxalate is consumed it binds up the calcium and reduces the adsorption soluble oxalates.