Given 72 $ 21 = 6 and 19 $ 42 = 4
The logic here is
$$\left(7+2\right)-\left(2+1\right)=6$$
$$\left(1+9\right)-\left(4+2\right)=4$$
The coded value is equal to the difference of sum of the digits of first number and sum of the digits of second number.
Similarly,
$$\left(1+8\right)-\left(2+6\right)=1$$
$$\therefore\ $$18 $ 26 = 1
Hence, the correct answer is Option C
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