Answer: A Contract was formed on February 5th
Step-by-step explanation:
The contract was formed the very day that Bob mailed Ann his acceptance which was on the 5th of February.
Ann attempted to revoke the acceptance too late as she did it a day after he had emailed his acceptance even though she only received it on the 7th.
The date she received the acceptance is of no consequence because this falls under the Posting Rule. This rule in Common Law countries essentially states an agreement is made as soon as the letter is posted even if it never gets to it's destination.