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B

Step-by-step explanation:

ABD (half of the rectangle) is a right-angled triangle.

so, Pythagoras applies.

c² = a² + b²

c is the Hypotenuse (the baseline opposite of the 90 degree angle) and in our case the line BD.

a and b are the 2 sides enclosing the 90 degree angle. in our case here the lines AD and AB.

so,

BD² = AD² + AB² = (6-1)² + (5-2)² = 5² + 3² = 25 + 9 = 34

BD = sqrt(34)