Given, 21 # 11 = 5, 18 # 4 = 7 and 6 # 2 = 2
The logic here is
$$\frac{21-11}{2}=5$$
$$\frac{18-4}{2}=7$$
$$\frac{6-2}{2}=2$$
The code value is the half the difference of the given numbers.
Similarly,
$$\frac{13-5}{2}=4$$
$$\therefore\ $$13 # 5 = 4
Hence, the correct answer is Option A
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