Explanation
To integrate a new dataset with pre-existing contacts, Northern Trail Outfitters should create a new data extension and link it to the other data extension. A data extension is a table that stores data in Marketing Cloud, such as contact information or purchase history. Data extensions can be linked by common fields or attributes using Contact Builder or SQL queries. References:
https://help.salesforce.com/s/articleView?id=sf.mc_es_data_extensions.htm&type=5

Explanation
To send a special discount offer to engaged customers on their email list, Northern Trail Outfitters should resolve how to determine engagement prior to generating the audience. Engagement is a measure of how customers interact with emails, such as opens, clicks, or conversions. Different criteria and thresholds can be used to define engagement, such as recency, frequency, or duration of interactions. Determining engagement will help Northern Trail Outfitters segment their audience and target their offer more effectively.
What the offer will be, when to send the email, and who the offer come from are important factors for designing and delivering the email, but they are not relevant for generating the audience based on engagement.
References: https://help.salesforce.com/s/articleView?id=sf.mc_es_engagement_split.htm&type=5

Explanation
To manually create a segment of both lead and contact records that can be targeted through the Send Flow in Content Builder, Northern Trail Outfitters should use a Salesforce Campaign. A Salesforce Campaign is an object in Sales Cloud that allows marketers to group leads or contacts for a specific marketing initiative, such as an email campaign. A Salesforce Campaign can be used as a target audience in Marketing Cloud using the Send Flow in Content Builder. References:
https://help.salesforce.com/s/articleView?id=sf.campaigns_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_co_send_flow.htm&type=5

Explanation
To send a welcome email to contacts when a contact record is added as a campaign member to a 'Loyalty Member' campaign in Sales Cloud, Northern Trail Outfitters should create a journey with a Salesforce Data entry source and an email activity. A Salesforce Data entry source allows marketers to inject contacts into a journey based on changes or events in Sales Cloud objects, such as campaign members. An email activity allows marketers to send an email message to contacts in a journey. References:
https://help.salesforce.com/s/articleView?id=sf.mc_jb_salesforce_data_event.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_jb_email.htm&type=5