Explanation

(c)
Starting with the letter A and arranging the other four letters, there are $4! = 24$ words. The starting with G, and arranging A, A, I, and N in different ways, there are $\frac{4!}{2!} = 12$ words. Next the 37^{th} word starts with I, there are 12 words starting with I. This accounts upto the 48^{th} word. The 49^{th} word in NAAGI. The 50^{th} word is NAAAIG