Answer:
C. Haven't
Explanation:
I figured this out by the process of elimination.
"They decided to have a wash have they." This sentence doesn't make sense because this is not the right word to fill in the blank.
"They decided to have a wash, didn't they." Also does not make sense because it is not the right type of past, present, or future tense.
"They decided to have a wash, haven't they." This is the only word that fits this sentence.
