Correct pair of primary standard substances:*NaOH, Oxalic acid, KMnO4Na2CO3, NaOH, KMnO4Na2CO3, Oxalic acid, Succinic acidKMnO4, HCl, oxalic acid

To determine the correct pair of primary standard substances, we need to consider their properties and uses in standardization processes.

1. NaOH and Oxalic acid: NaOH (sodium hydroxide) is commonly used as a primary standard substance for acid-base titrations. Oxalic acid is also a primary standard substance used for acid-base titrations. Therefore, this pair could be a correct option.

2. Na2CO3 and NaOH: Na2CO3 (sodium carbonate) is often used as a primary standard substance for standardizing acids. NaOH (sodium hydroxide) is also commonly used as a primary standard substance for acid-base titrations. Hence, this pair could also be a correct option.

3. Na2CO3 and Oxalic acid: Na2CO3 (sodium carbonate) is primarily used as a primary standard substance for standardizing acids. Oxalic acid is another primary standard substance used for acid-base titrations. Therefore, this pair could be a correct option as well.

4. KMnO4, HCl, and Oxalic acid: KMnO4 (potassium permanganate) is typically used as a primary standard substance for redox titrations. HCl (hydrochloric acid) is not commonly used as a primary standard substance. Oxalic acid, on the other hand, is used as a primary standard substance for acid-base titrations. Therefore, this pair is not a correct option.

Based on the information provided, the correct pair of primary standard substances could be NaOH and Oxalic acid or Na2CO3 and NaOH.