Final answer:
The correct answer is 'more quickly.'
Step-by-step explanation:
The correct answer to complete the sentence is A. more quickly.
In this sentence, we are comparing the rate of growth between James and Alex. To make this comparison, we need to use the comparative form of the adjective 'quickly.' 'More quickly' is the correct comparative form, indicating that James is growing at a faster rate compared to Alex.
For example, if James grows 5 centimeters in a month, and Alex grows 3 centimeters in the same time period, we can say that James is growing more quickly than Alex.
Learn more about Comparative form of adjectives