Explanation:

- The sentence with the underlined segment is: "The assignment was so difficult for Mohit to do on his own."

- Option 1 - was too difficult: This option is the correct substitute. It implies the assignment was beyond Mohit's ability to do alone, which aligns with the context.

- Option 2 - was so much difficult: This phrase is grammatically incorrect. "So much" does not pair with "difficult" properly.

- Option 3 - No substitution: As the original phrase can be improved for clarity, a substitution is needed.

- Option 4 - were so difficult: The subject "The assignment" is singular, so "was" is the correct verb form, not "were."