Which is the topic sentence of the following paragraph?

Determining the order of ideas in a paragraph is essential to producing a clear and understandable paragraph. A writer who aimlessly presents ideas the reader will not be very successful in presenting his or her ideas. Topic sentences help writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea. Adding support is key.

a)
Determining the order of ideas in a paragraph is essential to producing a clear and understandable paragraph.

b)
Topic sentences help writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

c)
A writer who aimlessly presents ideas the reader will not be very successful in presenting his or her ideas.

d)
Adding support is key.