Explanation
The correct answer is C because ensuring source channel is available in subscriber data can help NTO configure its welcome series in Journey Builder to honor the opt-in communication method. The source channel can be used as a decision split or an entry filter to send the appropriate message type (email or SMS) to each subscriber. Sending both email and SMS would not honor the opt-in communication method, but rather annoy the subscribers who only opted in for one channel. Creating one entry source for each messaging channel would not work because a journey can only have one entry source.