What triggers the core schema in Aaron Beck's cognitive therapy?

Critical incidents.

The correct answer is Option 3: Critical incidents.

Core schemas are deep-seated beliefs formed primarily during childhood experiences, but they remain latent (inactive). These schemas get activated or triggered when a person encounters a critical incident (such as failure, loss, rejection, or stress). Thus, Core schemas are triggered by critical incidents, though they are formed due to childhood experiences.

NCERT: "Another cognitive therapy is that of Aaron Beck. His theory of psychological distress characterised by anxiety or depression, states that childhood experiences provided by the family and society develop core schemas or systems, which include beliefs and action patterns in the individual. Thus, a client, who was neglected by the parents as a child, develops the core schema of “I am not wanted”. During the course of life, a critical incident occurs in her/his life. S/ he is publicly ridiculed by the teacher in school. This critical incident triggers the core schema of “I am not wanted” leading to the development of negative automatic thoughts. Negative thoughts are persistent irrational thoughts such as “nobody loves me”, “I am ugly”, “I am stupid”, “I will not succeed”, etc. Such negative automatic thoughts are characterised by cognitive distortions. Cognitive distortions are ways of thinking which are general in nature but which distort the reality in a negative manner. These patterns of thought are called dysfunctional cognitive structures. They lead to errors of cognition about the social reality."