How to answer the question

Each of the numbers in the second row is a square. The numbers are the squares of 1, 4, 9, 16.
Each of those numbers, in turn, is the square of the number in the first row.
That makes the number in the second row be the 4th power of the number in the first row.
The n-th term is n⁴.
2 * 2 * 2 * 2 = 2^4 = 16
3 * 3 * 3 * 3 = 3^4 = 81
4 * 4 * 4 * 4 = 4^4 = 256
5 * 5 * 5 * 5 = 5^4 = 625
Each time, the number in the top row is raised to the 4th power.
The pattern is for a number n in the top row, you get n^4 in the bottom row.