Answer :
The word which can be best used to complete the sentence is "gradual".
What is Word Choice?
This refers to the selection of words which a writer makes use of while writing and this is effectively a diction.
With this in mind, we can see that the given sentence talks about a person almost not noticing the change from spring to summer because the summer was so gradually changing.
Read more about vocabulary here:
https://brainly.com/question/25567167