Given an n-element array X, algorithm D calls algorithm E on each element X[i]. Algorithm E runs in O (i) time when it is called on element X[i]. What is the worst-case running time of algorithm D?

O (n^2)

Explanation:

The number of elements in the array X is proportional to the algorithm E runs time:

For one element (i=1) - > O (1)

For two elements (i=2) - > O (2)

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For n elements (i=n) - > O (n)

If the array has n elements the algorithm D will call the algorithm E n times, so we have a maximum time of n times n, therefore the worst-case running time of D is O (n^2)