Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
response [rɪsˈpɔns] n (answer) отве́т; (reaction) реа́кция
in response to (your letter) в отве́т на (Ва́ше письмо́)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.