In triangle ABC , angle A is 60 degrees and $$\mathrm{AB}=\mathrm{AC} \Rightarrow \angle \mathrm{B}=\angle \mathrm{C}=\frac{1}{2}(180-60)=60$$ (When sides are equal, opposite angles are also equal and the sum of the angles of a triangle is 180 degrees). So, triangle ABC is an equilateral triangle.

Also we are given that a semi circle is drawn with diameter along the side $$B C$$, so the radius of the circle, say $$r$$, must be half of $$B C$$

The length of the arc of the semi circle $$=\frac{1}{2}(2 \pi r)=\pi r=50 \pi \Rightarrow r=50$$, so we get $$B C=$$ $$2 \mathrm{r}=2 \times 50=100$$

Finally, the perimeter of the triangle ABC is $$3 \times$$ Side $$=3 \times 100=300$$
Hence the answer is (C).