Please help me to solve this - Given an (unsorted) array of n elements, can we Sort them in O(n) time?

it depends. radix and counting sort work in O(n) but they require special constraints on input like the numbers are in range 1 to n etc.there are some other sorts but they require special hardware requirements. for practical purposes O(nlogn) in worst case is the best we know.

For details check out the wikipedia page.

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No, there is no algorithm that sorts in O(n) time unless, the array is sorted or almost sorted.

In that case algorithms like Tim Sort and Bubble Sort give a best-case time complexity of O(n)

In general no. O(n\log(n)) is a tight bound (assuming element comparison in constant time). But for special cases it is possible. If you have an array of integers bounded by O(n) or a string (considering it to be an array of chars) you can use Counting sort.