The correct answer is A , because:

In MobileConnect and Journey Builder SMS activities, phone number normalization ensures that mobile numbers are automatically formatted to include the appropriate country code (such as "+1" for the United States) based on a selected default country.

If the phone numbers coming from Salesforce (or any source) do not include a country code, enabling "Normalize Phone Number" in the Journey settings corrects this by automatically appending the country code according to the selected settings.

Salesforce official documentation states:

" Normalize Phone Number ensures that phone numbers missing a country code are automatically formatted based on a selected default country. This improves message deliverability in SMS activities within Journey Builder."

(Source: Salesforce Help - MobileConnect and Journey SMS Send Configuration)

✅ In this case:

Without normalization, SMS providers expect numbers in E.164 format (which requires the country code, e.g., +1 for US numbers).

If the number is missing the "+1", the message fails to send.

Enabling Normalize Phone Number in the Journey SMS settings fixes the issue without needing to reformat Salesforce records manually.

Why the other options are incorrect:

❌ B. Format the phone field in Salesforce as +1[phonenumber] :

While correct formatting helps, it requires manual mass update of existing records and doesn't automatically handle new ones, making it inefficient.

❌ C. Select 'US' as the sending country in SMS activity :

Selecting the sending country affects short code usage , not the normalization of incoming phone numbers.

❌ D. Configure the phone field in Salesforce as US locale :

Locale settings don't automatically add a "+1" country code to phone numbers; normalization is still needed.

Reference Extract from Salesforce MobileConnect documentation:

"Use Normalize Phone Number to correct phone number formatting errors for Journey Builder SMS sends. Automatically prepends country codes based on the specified country when absent."

(Source: Salesforce Marketing Cloud MobileConnect User Guide)

Thus, the correct answer is A .