The given reaction is:
Mn2+ + KNO3 + NaOH → MnO2 + KNO2 + NaNO3 + H2O

Step 1: Balancing the equation
2Mn2+ + 10KNO3 + 2NaOH → MnO2 + 5KNO2 + NaNO3 + H2O

Step 2: Determining the oxidation state of Mn
In the reactant side, the oxidation state of Mn is +2. In the product side, the oxidation state of Mn is +4. Hence, the oxidation number of Mn changes from +2 to +4.

Step 3: Understanding the reaction
When a manganous salt (Mn2+) is fused with a mixture of KNO3 and solid NaOH, a redox reaction occurs.

Step 4: Analyzing the reactants and products
- Manganous salt (Mn2+) is an ionic compound containing Mn2+ ions. The oxidation state of Mn in this compound is +2.
- KNO3 is an ionic compound containing K+ and NO3- ions. It does not undergo any change in oxidation state during the reaction.
- NaOH is a strong base and does not undergo any change in oxidation state during the reaction.
- MnO2 is a compound containing Mn in the +4 oxidation state.
- KNO2 is a compound containing K+ and NO2- ions. It does not undergo any change in oxidation state during the reaction.
- NaNO3 is an ionic compound containing Na+ and NO3- ions. It does not undergo any change in oxidation state during the reaction.
- H2O is a compound where the oxidation state of H is +1 and O is -2.

Step 5: Analyzing the reaction mechanism
In this reaction, Mn2+ is oxidized to Mn4+. The KNO3 and NaOH act as oxidizing agents, causing the oxidation of Mn2+. The Mn2+ ions lose electrons and are oxidized to Mn4+ ions. This is evident from the change in the oxidation state of Mn from +2 to +4.

Conclusion
The oxidation number of Mn changes from +2 to +4 when a manganous salt is fused with a mixture of KNO3 and solid NaOH. Therefore, the correct answer is option 'C' (6).