**Concept: The question tests on Combinatorics**

**Solution:**

We can use either the box method or by using the equation for arrangements.

The number of ways of arranging n distinct things taken r at a time is nPr = n! / (n - r)!

So 10 distinct letters taken 5 at a time = 10P5 = 10!/5!

= (10 * 9 * 8 * 7 * 6) * 5! / 5!

= 10 * 9 * 8 * 7 * 6 * 5

Option B