Correct Answer: certain

Solution : The most appropriate antonym of the word "suspicious" in the given sentence is the first option.

Explanation for each option:

1. "Certain" - This is the correct antonym for "suspicious" as it represents a lack of doubt or uncertainty, which contrasts with being suspicious.

2. "Problematic" - This word is not an antonym but relates to the issue with the web server, and it does not contrast with "suspicious".

3. "Submit" - It is a verb with several meanings, often relating to yielding to authority, presenting something for consideration.

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4. "Portal" - It commonly refers to an entry point, whether physical, digital, or conceptual, providing access to something else.

So, the correct sentence would be: "Although the web server was problematic, Sudha was certain that she will submit her form on the portal by evening."