Option 4 : 5

**Given:**

The given expression is 1015 × 521 × 739

**Concept used:**

The unit digit of 1015 = 1 (The unit digit of multiple of 1 will always be one)

The unit digit of 521 = 5 (The unit digit of multiple of 5 will always be 5)

The unit digit of 7 follows the cycle of 4

For 739,

Unit digit of 71 = 7

Unit digit of 72 = 9

Unit digit of 73 = 3

Unit digit of 74 = 1

Unit digit of 75 = 7

We see from the above process; the unit digit is repeating after 4th power.

⇒ Unit digit of (74 × 9) × 73 = 3

⇒ 1015 × 521 × 739 = 1 × 5 × 3 = 15

**∴ 5 is the unit digit of the given expression.**