C. a=10 √3, b=5 √3, c=15, d=5
The short cut for a 30 60 90 triangle is that the hypotenuse is 2 times short leg. So you divide 10 by 2 to get 5 as the short leg.
Another short cut is that the long leg is the short leg times √3. 5 times root 3 equals to 5 √3. Making b equal to 5 √3