Discussion

Explanation:

Unit digit of (75)²⁴² = 5.

Power of any number with 5 at the unit’s place will always have a 5 at its units place, irrespective of the positive natural number power.

Also, last digit of 39⁹⁸ = last digit of 9⁹⁸ = last digit of 9² = 1.

Now, last digit of 72¹²¹ = last digit of 2¹²¹ = last digit of 2¹ = 2.

So, basically after every 4^th power, the units digit repeats itself.

Thus, the digit at the unit’s place is 5 + 1 + 2 = 8.

Hence, option (a).

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