PQRSTU is definitely divisible by 4.
Since Sonal and Gaurav have 2 numbers written on a piece of paper, and Sonal's number PQR is divisible by 3 and 4, while Gaurav's number STU is divisible by 2 and 9, and the two numbers are joined to form a new 6- digit number PQRSTU, to determine the number by which the number PQRSTU is definitely divisible by, the following logical reasoning must be performed:
Since the numbers are not added mathematically, but joined numerically, and that STU is the last number in the row, and that it is divisible by 3 and 4, while only 4 is among the options, the number PQRSTU is definitely divisible by 4.
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