Let us first put the given data in the form of a diagram.
We know that tangents drawn from an external point will be equal.
Therefore,
AD = CD
Since CD is given as 4 cm,
AD = 4 cm
Similarly,
BD = CD
Therefore,
BD = 4 cm
AB = AD + BD
AB = 4 + 4
AB = 8 cm
Therefore, the answer to this question is option (c).