Answer:
because of
Explanation:
→sumeena couldn't attend her class because of her illness => yes she is ill, so she couldn't go to school. in this sentence, ''because of'' is similar with ''reason of''.
→Therefore is similar with ''so''. Sumeena couldn't attend her class therefore her illness. => When you don't attend your class, you become ill ? Nooo, it's nonsense.
→if she attend her class in spite of her illness, we can use 'in spite of'. but she couldn't attend her class
Hope this helps ^-^
