To find out how much grain the farmer can hold we need to find the volume of the silo. The silo is made from a circular cylinder and half a sphere.
The volume of the cylinder is given by:

In this case r=10 and h=30, plugging this values in the equation above we have that the volume of the cylinder is:

Hence the volume of the cylinder is 9420 cubic feet.
Now we need to find the volume ot half the sphere, the volume og an sphere is given by:

Plugging the radius we have that:

Now, we only need half this volume then the volume of the top of the silo is:

Finally we add the volume of the cylinder and half the sphere, therefore the volume of the sylo is:

and the answer is B.