Queries should return less than ~30% of records to be selective.

Creating a custom object related to the Invoice object with a picklist field for the Type allows the architect to categorize the attachments and report on them by Type. The standard Attachment object does not have a ContentType picklist field, and adding a custom picklist field to it would not be best practice.

The correct answer is C, leverage Salesforce Connect to bring the order line item from the legacy system to Salesforce. Salesforce Connect is a feature that allows you to integrate external data sources with Salesforce and access them in real time without copying or synchronizing the data.
This way, the sales team can view the order status from the ERP system without having access to it. Leveraging Canvas, building batch jobs, or building real-time integration are also possible solutions, but they are more complex and costly than using Salesforce Connect.

According to the Salesforce documentation, duplicate leads are leads that have the same or similar information as other leads in Salesforce. Duplicate leads can cause poor customer experience, higher costs, and inaccurate reporting. To mitigate the issues caused by duplicate leads, some of the recommended practices are:
Standardize lead information across all channels (option B). This means using consistent formats, values, and fields for capturing lead data from different sources, such as web forms, email campaigns, or third-party vendors. This can help reduce data quality issues and make it easier to identify and prevent duplicate leads.
Implement a third-party solution to clean and enrich lead data (option D). This means using an external service or tool that can validate, correct, update, and enhance lead data before or after importing it into Salesforce. This can help improve data quality and accuracy, and reduce duplicate leads.
Implement a de-duplication strategy to prevent duplicate leads (option E). This means using Salesforce features or custom solutions that can detect and block duplicate leads from being created or imported into Salesforce. For example, using Data.com Duplicate Management, which allows defining matching rules and duplicate rules for leads and other objects.

The best way to implement the storage of master key within Salesforce is to store it on the contact object as an external ID field for referential imports. This way, the data architect can use the master key as a unique identifier to match records from different systems and avoid duplicates. The other options are not feasible because they either require additional storage or do not support referential imports.