Why does this phrase use “de” and not “o” “te wo furu hito ni egao de kotae”?

Question

This phrase "te wo furu hito ni egao de kotae" is translated as "Smiling back to the people who waves their hands"

"te wo furu hito ni" I suppose it's "to the people who waves their hands" , kotae is "to respond" and "egao" is "smile". Then why it's "egao de kotae" and not "egao o kotae" . With "egao de kotae" isnt it trying to say "respond a smile" or the literal translation would be another? Does this verb simply work with "de" instead of "o" or is there any other explanation and I'm getting it all wrong?

Answer 1

笑顔{えがお} is not the object of 答える, instead it's showing the manner in which the answer was given, "with a smile", hence the use of the particle で.　I don't believe that 答える is a transitive verb in Japanese. You can use it with either に or と such as

はいと答えた。 She answered "yes".

or

質問に答えた。 She answered the questions.

But I don't believe it's grammatically correct or natural to say

** 質問を答えた。

Answer 2

English is off-topic, but I can't comment yet, so I'll note by answer that the preferred English would be "responding with a smile".

So the English and Japanese are somewhat corollary here.