Explanation:

To find the correct word for filling in the blank we must know the meaning of the words given as options.

1. felicitate - congratulate.

2. complicate - consisting of many interconnecting parts or elements; intricate.

3. confuse - make (someone) bewildered or perplexed.

4. facilitate - make (an action or process) easy or easier.

Hence after looking at the meanings it is clear that the word 'facilitate ' is the most suitable word for filling in the blank. So option 4 is the correct answer.