The best meaning for the underlined idiom about the speed the grandpa and grandma showed up is Instantly.
The idiom used is "at the drop of a hat" and it is used to refer to something happening really fast or instantly.
Saying the grandpa and grandma showed up in such a manner means that they came as soon as they heard the narrator was sick.
The question is:
What is the meaning of the underlined idiom?
Find out more on idiomatic expressions at https://brainly.com/question/902417.
#SPJ1