The Chief Executive Officers (CEOs) from two different companies are discussing the highly sensitive prospect of merging

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The Chief Executive Officers (CEOs) from two different companies are discussing the highly sensitive prospect of merging their respective companies together.
Both have invited their Chief Information Officers (CIOs) to discern how they can securely and digitally communicate, and the following criteria are collectively determined:
✑ Must be encrypted on the email servers and clients
✑ Must be OK to transmit over unsecure Internet connections
Which of the following communication methods would be BEST to recommend?

A. Force TLS between domains.
B. Enable STARTTLS on both domains.
C. Use PGP-encrypted emails.
D. Switch both domains to utilize DNSSEC.