Hey there!
This link is All About the topic sentence of a paragraph:
http://www.csun.edu/~hcpas003/topic.html
Hope it helps!
The correct answer is option two.
An infinitive phrase consists of an infinitive verb, and can be followed by a complement or a modifier, which is usually an adverb.
In the sentence above, the infinitive phrase comes before a complement "variety and interest to their writing."
Farther! Cuz the other options don't make sense grammatically.
Got any more details id be happy to help