49 C.F.R. § 209.107 – Reply.

(a) Within thirty (30) days of the service of a notice of probable violation issued under § 209.105, the respondent may—
1. (1) Pay as provided in § 209.109(a) and thereby close the case;
2. (2) Make an informal response as provided in § 209.111; or
3. (3) Request a hearing as provided in § 209.113.
(b) The Chief Counsel may extend the thirty (30) days period for good cause shown.
(c) Failure of the respondent to reply by taking one of the three actions described in paragraph (a) of this section within the period provided constitutes a waiver of his or her right to appear and contest the allegations and authorizes the Chief Counsel, without further notice to the respondent, to find the facts to be as alleged in the notice of probable violation and to assess an appropriate civil penalty.