The feature that can be used to meet these requirements is two-step approval process. An approval process defines the steps and criteria for approving records. A two-step approval process requires two levels of approval before a record can be deleted. The app builder can configure an approval process for opportunities that requires sales reps to submit their delete requests for approval from their managers. Option A is incorrect because Process Builder with Submit for Approval action is not a valid feature, as Process Builder cannot submit records for approval. Option B is incorrect because Approval Process with a triggered Flow process is not a valid feature, as Approval Process cannot trigger Flow processes. Option C is incorrect because Approval Process with Time-Dependent Workflow action is not a valid feature, as Approval Process cannot use Time-Dependent Workflow actions.

Creating a unique External ID field on the custom object and using Data Loader to upsert records prevents duplicates and ensures correct record updates by matching the unique ID.
Reference:
Salesforce Help - External ID

Preexisting information will remain even if it was over 255 characters if the app builder tries to convert a preexisting long text area field to text area. However, users will not be able to edit or save records with more than 255 characters in that field until they reduce the number of characters. Preexisting information will not truncate, be lost, or cause an error message.

To ensure process compliance where cases cannot revert to an earlier status and certain fields are required based on specific case criteria, the recommended solution is:
Configure validation rules with help text (A). Validation rules enforce data integrity and business processes by preventing users from saving records if certain conditions are not met. For instance, a validation rule can prevent the status of a case from being changed back to an earlier status and can conditionally require fields based on other field values on the case.
Dependent picklists (B) ensure field values depend on another field's value but do not enforce the inability to revert status. Approval processes (C) are used for step-by-step record approval and are not designed to prevent status reversion or conditionally require fields. Making fields required on the page layout (D) ensures they are always required when accessing the record through that layout but does not enforce conditional requirements based on other criteria.
Reference for creating validation rules in Salesforce:
Validation Rules: https://help.salesforce.com/articleView?id=sf.validation_rules.htm&type=5

The app builder should provision a Partial Data sandbox to the developer. A Partial Data sandbox can have up to 5 GB of data, which meets the requirement of having at least half a gigabyte of data. A Partial Data sandbox can also be refreshed every 5 days, which meets the requirement of refreshing after each three-day sprint. Option A is incorrect because a Developer sandbox can only have up to 200 MB of data, which does not meet the requirement. Option B is incorrect because a Full Copy sandbox can only be refreshed every 29 days, which does not meet the requirement. Option C is incorrect because a Developer Pro sandbox can only have up to 1 GB of data, which may not be enough for the requirement.