Explanation:

✅ C. SET @firstName = AttributeValue("First Name")
This is the recommended and safest method to retrieve a value from a Sendable Data Extension inside AMPscript. AttributeValue() retrieves the field based on subscriber context and handles special characters, spaces, and fallback logic gracefully. It avoids runtime errors when the field doesn't exist.

❌ A. SET @firstName = [First Name]
This syntax works in inline AMPscript, typically used inside HTML content like:
%%[ SET @firstName = [First Name] ]%%
While it is valid, it’s not recommended for values with spaces or special characters in the field name. It can fail if not properly escaped, so AttributeValue() is safer.

❌ B. SET @firstName = %%First Name%%
This syntax is incorrect for setting a variable. %%First Name%% is a content substitution tag used in HTML output, not within AMPscript logic. You can display a field this way, but not assign it to a variable.

❌ D. SET @firstName = "First Name"]
This is invalid syntax in AMPscript. The brackets don't match, and the field is treated as a plain string, not as a variable or DE field. This would cause a parsing error.