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Consider the following schedule:  $S =$ ${R1(A),R2(B),W2(A),W1(A)}$
Assume that Transaction $T1$ has started execution before the Transaction $T2$.

Then, the schedule is:

  1.      Allowed under Basic Timestamp Protocol, but not under Thomas Write Rule.
  2.      Not allowed under Basic Timestamp Protocol, but allowed under Thomas Write Rule.
  3.      Allowed under both Basic Timestamp Protocol and Thomas Write Rule.
  4.      Not allowed under both Basic Timestamp Protocol and Thomas Write Rule.
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Thomas write rule says : " It states that, if a more recent transaction has already written the value of an object, then a less recent transaction does not need perform its own write since it will eventually be overwritten by the more recent one. "

as transaction T1 has started execution before the Transaction T2 then the schedule  is allowed under Tomas write rule.

Basic timestamp rule says : " the transaction started before the object's write timestamp it means that something changed the object's data after the transaction started. In this case, the transaction is canceled and must be restarted. " so T1 is started R1(A) before W1(A) hence it is canceled .  This schedule is Not allowed under Basic timestamp protocol

hence option B is correct .

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