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Answer A: 100 Deg.

Explanation:

Because angle ADC is 40 degrees, angle CAD is also 40 degrees. This is because lines CD and CA are the same length.

Angle CAD is equal to Angle BCA because they are vertical angles. This makes angle BCA 40 degrees.

Angle BAC is equal to BCA because the lines BA and BC are equal. This makes angle BAC 40 degrees.

You now know 2 angles of the triangle ABC. Knowing that all angles in a triangle add up to 180 degrees, subtract the other two angles to get angle ABC. 180-(40+40)= 100. This makes angle ABC equal 100 degrees