Suppose you do not know the value of k$.$ Use set.seed$(123)$ to select $80\%$ of data for training and $20\%$ for validation. Search the optimal k in $1,3,5,7,9,11,13,15,17,19.$

What is the optimal k$?$

Group of answer choices

$9$

$5$

$7$

$11$

$17$