The correct answer is:
A. wave velocity increases.
When the tension in a spring (or a stretched string) is increased, the wave velocity on that spring also increases. This is because the wave velocity \( v \) in a medium is given by the formula:
\[ v = \sqrt{\frac{T}{\mu}} \]
where \( T \) is the tension in the medium and \( \mu \) is the mass per unit length. An increase in tension \( T \) leads to an increase in wave velocity \( v \) if the mass per unit length \( \mu \) remains constant.