Final answer:
On a timeline, a milestone 13 years in the future is to the right of a milestone 4 years in the future and to the left of a milestone 19 years in the future.
Step-by-step explanation:
On a timeline, events are typically arranged from past to future, generally depicting time progressing from left to right. Therefore, a milestone that is 13 years in the future would be positioned to the right of a milestone that is 4 years in the future, since 13 years is further ahead in time than 4 years. Similarly, the 13-year milestone will be to the left of a milestone that is 19 years in the future because 13 years is less far ahead than 19 years. Hence, the correct answer is 'a. left; right.'