The opportunity cost of $1$ unit of clothing is:

Clothing

Andy

$10$

$10$

Debbie

$20$

$30$

$1$ unit of food for Andy and $3/2$ units of food for Debbie

the same for both Andy and Debbie

$1$ unit of food for Andy and $\frac{2}{3}$ units of food for Debbie

impossible to determine from the information provided