The question asks for the major product in a mononitration reaction, but the crucial reaction scheme and the structures of options A, B, C, D are missing from the prompt.

Without the substrate structure, it is impossible to determine:

1. Which ring or carbonyl-containing compound is being nitrated
2. Whether substitution occurs on an aromatic ring or elsewhere
3. The directing effects of substituents such as $-CHO$, $-COR$, $-COOH$, alkyl, halogen, etc.
4. Which option corresponds to ortho/meta/para substitution

For nitration in organic chemistry, the answer depends heavily on substituent effects:

• $-CHO$, $-COR$, $-COOH$, $-COOR$, $-CN$, $-NO_2$ are generally deactivating and meta-directing.
• Alkyl groups are generally ortho/para-directing.
• Halogens are deactivating but ortho/para-directing.

So the major product can only be identified if the original compound and the option structures are visible.

Conclusion

Because the reaction diagram and option structures are absent, the problem is underdetermined from the provided text alone. Therefore, I cannot independently verify whether option A is correct.

Please provide the reaction figure (substrate and options), and I can solve it completely step-by-step.