Step 1: Concept
The argument of a trigonometric function must be completely dimensionless, meaning $[\alpha t] = M^0L^0T^0$. Furthermore, the overall trigonometric function component $\cos(\alpha t)$ itself is dimensionless.
Step 2: Meaning
Since the argument is dimensionless, the dimension of $\alpha$ must be the reciprocal of time: $[\alpha] = \frac{1}{[t]} = T^{-1} = M^0L^0T^{-1}$.
Step 3: Analysis
For the coefficient $\beta$, we equate its dimensions to the remaining terms: $[\beta] = \frac{[F]}{[v]^2}$. Substituting the standard dimensional formulas for force ($[F] = MLT^{-2}$) and velocity ($[v] = LT^{-1}$):
$[\beta] = \frac{MLT^{-2}}{(LT^{-1})^2} = \frac{MLT^{-2}}{L^2T^{-2}} = ML^{-1}T^0$.
Step 4: Conclusion
Combining both results gives $[\alpha] = M^0L^0T^{-1}$ and $[\beta] = ML^{-1}T^0$, which precisely matches option (C).
Final Answer: (C)