Final answer:
The linking word 'Previously' fits best in the blank, as it establishes a time contrast, indicating that the ability to draw a straight line is a development from the past to the present.
Step-by-step explanation:
The linking word that best fills the blank in the sentence ", I couldn't draw a straight line." is Previously. This is because 'previously' implies that the statement that follows is in contrast with a past condition or ability. The other options, 'In conclusion' and 'Until', do not logically fit the context of the sentence. 'In conclusion' is typically used to summarize arguments or ideas at the end of a piece of writing, while 'Until' conveys the idea of something happening up to a specific point in time, which does not match the context required here.
In the context of transitional words and phrases, using Previously effectively creates a contrast between the past and the present. It informs readers that there has been a change in the situation or skill level of the subject over time.