In which file will the event observer be declared?

You have created a custom module which must perform an action immediately after an order is placed, but only on the store front of the merchant site. You have selected the checkout_submit_all_after as the target event which the module will observe.

In which file will the event observer be declared?
A . etc/frontend.xml
B . etc/events.xml
C . etc/config.xml
D . etc/frontend/events.xml

Answer: D