Final answer:
David Donovan's acceptance mailed on June 8 meets the June 10 deadline based on the mailbox rule, creating a valid acceptance.
Step-by-step explanation:
When Helene Black's offer to David Donovan stated the offer was "subject to receipt of acceptance by June 10," a specific deadline for acceptance was set. In contract law, if an acceptance is mailed before the deadline, it is typically considered valid when sent, not when received. This is known as the mailbox rule. Since Donovan mailed his acceptance on June 8, it falls within the deadline even though it did not reach Black until June 11. Therefore, there was a valid acceptance of the offer.