You want your module to set some data on the shopping cart item when a product is added to the cart on the
frontend. Which event can be used regardless of how the product is added to the cart (from wishlist, through
quote merge, etc.)?

A.
checkout_cart_add_product_complete

B.
checkout_cart_product_add_after

C.
checkout_cart_save_before

D.
sales_quote_add_item

E.
sales_quote_product_add_after

F.
sales_quote_merge_after