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We can use a n size queue.
Keep filling the queue. Once the queue is full then keep adding the next lines and remove the front element from the queue at the same time.
So, at any instance the queue will give the last n rows read from the file.

Yes we should free the memory of the front element when removing it from the queue. So, that only n rows are always present in the memory.

One more approach will be similar to printing the last n node of a single linked list. Use 2 file pointers(fp1 and fp2). Move the first pointer(fp1) n rows or till the end-of-file whichever comes first. If there are more that n lines in the file then start moving the 2 file pointers till fp1 reaches the end of file. Once fp2 reaches end of file, then move fp1 till the end of file printing every line and this gives the last n lines in the file.

fill an array of n Strings in cyclic way until you reach EOF.
then traverse from the cyclic beggining and print.

e.g. array[i % n] <= read
etc.