Comparing different asymptotic notations
Well, we know how the algorithm C running time grows - it's linear, and it's two-side (lower and upper) bound.

However there is still not enough information to choose the best (meaning: fastest in practice) algorithm here, because:


We know only upper bounds for algorithms A and B - they might be more efficient than C actually, we just don't know that yet.
We don't know constants, hidden in the $\Theta$ bounds - they might make the algorithm C less practical than A or B for limited problem size.


Using the given information we can say only that for some sufficiently large problem size the algorithm C might win over algorithms A and B.