We are given a rhombus with diagonals of lengths 12 cm and 16 cm. To find the length of the side of the rhombus, we can use the property of the rhombus that states:
\[
\text{Side}^2 = \left(\frac{d_1}{2}\right)^2 + \left(\frac{d_2}{2}\right)^2
\]
where \( d_1 \) and \( d_2 \) are the lengths of the diagonals.
Step 1: Apply the formula.
Substitute the values for the diagonals \( d_1 = 12 \) cm and \( d_2 = 16 \) cm:
\[
\text{Side}^2 = \left(\frac{12}{2}\right)^2 + \left(\frac{16}{2}\right)^2
\]
\[
\text{Side}^2 = 6^2 + 8^2 = 36 + 64 = 100
\]
Step 2: Solve for the side.
Taking the square root of both sides:
\[
\text{Side} = \sqrt{100} = 10 \, \text{cm}
\]
Step 3: Conclusion.
Thus, the length of the side of the rhombus is 10 cm. The correct answer is (B).