Answer: E . 7

Justification:

Clearly, unit's digit in the given product = unit's digit in 7¹⁵³ x 1⁷².
Now, 74 gives unit digit 1.
7¹⁵² gives unit digit 1,
.`. 7¹⁵³ gives unit digit (1 x 7) = 7. Also, 1⁷² gives unit digit 1.
Hence, unit's digit in the product = (7 x 1) = 7.