Let's solve this,
= √5 . √15
- "." here means multiply, am sure u know
= √5 × √3 × 5
- constituent of 15 is 3 × 5
= √5 × 3 × 5
- "√" can be taken common
- as there is a pair of 5, which means it is a square we can take it out of the sqaure root.
= 5√3
And our answer is done, option 5√3! :P