In fact, checking whether an array is sorted can be done in time $O(n)$, and there are efficient (polynomial time) algorithms for computing the maximum flow as well. On the other hand, the traveling salesperson problem is known to be NP-hard, as is the knapsack problem.
However, that is not what your question is about. Finding the maximum flow is not a decision problem, and so it is not eligible for being in NP. All other problems are decision problems, and as the preceding paragraph demonstrated, they are all in NP (one in them is even in P, and the other are NP-complete).