Answer:
The correct option is A.
Explanation:
The area of a triangle is
![A=(1)/(2)* base* height](https://img.qammunity.org/2020/formulas/mathematics/middle-school/cbann11bggh9e26xp38tutpvnvolfo6lhd.png)
From the given figure it is clear that the base of the triangle STR is 6 ft and the height of the triangle 10 ft. So, the area of triangle STR is
![A=(1)/(2)* 6* 10](https://img.qammunity.org/2020/formulas/mathematics/middle-school/tz5mhxpng21uojmoc4srabtqnqts9q1zli.png)
![A=3* 10](https://img.qammunity.org/2020/formulas/mathematics/middle-school/gqrxfe1grij606hxsqsliuxtl9ifds32jb.png)
![A=30](https://img.qammunity.org/2020/formulas/mathematics/middle-school/u09m7xebgah4by4e0bweb1aeru8mbana4x.png)
The area of triangle STR is 30 square ft. Therefore the correct option is A.