Comprehensive and Detailed Explanation:

The correct answer is A — Orders should be set up to allow injection upon update in Connect Configuration.

When setting up Salesforce Data Entry Source in Marketing Cloud Connect, you must explicitly configure whether records should inject only upon creation or also upon updates.

If injection on update is not configured, only the original create event triggers the Journey entry.

❌ Why others are wrong:

B: The associated Contact does not need to be updated — it’s the Order object that triggers the injection.

C: Synchronized Data Extensions are not used here — it’s a direct connection via Marketing Cloud Connect, not polling.

D: Even if status fields change properly, if updates are not allowed to trigger injection, no entry will occur.

Exact Extract from Salesforce Official Documentation:

Salesforce Help — Salesforce Data Entry Configuration:

"When configuring Salesforce Data Entry Sources, ensure the setting to allow journey entry on record update is enabled if updates to the record should trigger journey entry."

(Source: https://help.salesforce.com/s/articleView?id=sf.mc_jb_salesforce_data_entry_source_settings.htm )