Here, the "non-serial" in "non-serial interleaving" is superfluous (redundant). Interleaving, by its very nature, is non-serial.
You've been asked to interleave, which means, you cannot execute all operations of one transaction in a row followed by all operations of the other transaction. Otherwise it would just be a serial schedule, which we don't want.
Our schedule can start with the first operation of either T1 or T2 (it has to start somewhere). If you start with T1 in your schedule (read1 P) then you cannot end T1 (write1 Q) without first starting T2 (read2 Q). Similarly, if you start with T2 (read2 Q) then you cannot end T2 (write2 P) without first starting T1 (read1 P). In both cases, you are taking a serial(izable) schedule (T1 $\rightarrow$ T2 in the first case, and T2 $\rightarrow$ T1 in the second) and changing the order of two conflicting operations.
We know that the moment we change the order of a pair of conflicting operations in a serial (or serializable) schedule , the schedule no longer remains conflict serializable. Hence, B is the correct answer.