Final answer:
The 'Time is of the essence clause' in a real estate contract requires parties to proceed in good faith and not delay the process. It signifies that deadlines within the contract must be met, otherwise it might constitute a breach.
Step-by-step explanation:
The clause in a real estate sales contract that directs the parties to proceed in good faith and not unduly delay the process is the Time is of the essence clause. This clause emphasizes that the timing of obligations such as the closing date and other deadlines in the contract are critical, and any delays can be considered a breach of contract, potentially allowing the non-breaching party to seek remedies. None of the other options provided, which include Acceleration clause, Indemnification clause, or Severability clause, directly refer to the adherence to deadlines or performance times within a contract.