Step 1: Concept
The speed $v$ of a transverse wave on a string is given by $v = \sqrt{\frac{T}{\mu}}$.
Step 2: Analysis
Speed is directly proportional to the square root of tension ($v \propto \sqrt{T}$).
Step 3: Calculation
If tension becomes $2T$, the new speed $v' = \sqrt{2} \times v$.
Final Answer: (C)