The most common contraindications for Catlan's appliance are:
Existing anterior open bite
Periodontally compromised mandibular anterior teeth
Cases where further proclination of maxillary anterior teeth is undesirable (e.g., severe anterior flaring if the goal is to reduce it).
Severe skeletal discrepancies.
Comparing the options:
A. Posterior open bite: Not a direct contraindication for using Catlan's, but Catlan's doesn't treat it and could lead to anterior open bite.
B. Class I with anterior flaring: Catlan's causes flaring, so contraindication if flaring is the problem.
C. Severe class III with reverse overjet: Catlan's is not for severe skeletal problems.
D. Anterior open bite: Strong contraindication.
The question is effectively asking for the case where it can be used. If the question implies that the appliance is generally contraindicated in all but one of the listed situations.
Given that Catlan's appliance creates an anterior bite plane, which effectively creates a temporary posterior open bite to allow the anterior crossbite correction, a pre-existing posterior open bite is not an absolute contraindication in the same way an anterior open bite is. It simply means the appliance isn't addressing the posterior open bite.
Therefore, the most logical answer as the "EXCEPT" case (where it can be used, or is not contraindicated) is A.