What is the unit digit in the multiplication of 1 × 3 × 5 × 7 × 9 × ... × 999?

The correct answer is Option 5.

Step 1: Analyze the given expression
We are asked to find the unit digit of the product of odd numbers:
$1\times 3\times 5\times 7\times 9\times \cdots \times 999$

Step 2: Understand the property of multiplication by 5
Notice that one of the factors in this product is 5.

When 5 is multiplied by any odd integer, the resulting number always ends with the unit digit 5:
$5\times 1=5$
$5\times 3=15$ (unit digit is 5)
$5\times 7=35$ (unit digit is 5)
$5\times 9=45$ (unit digit is 5)

Step 3: Conclusion
Since all numbers in the given series are odd numbers, multiplying 5 by any combination of these odd numbers will always yield a product ending with the unit digit 5.

Therefore, the unit digit in the given multiplication is 5.