Which of the following describes a communication disorder?

A communication disorder is characterized by difficulties in verbal or non-verbal communication, impacting an individual's ability to convey thoughts, ideas, or emotions effectively. In this context, the option indicating an inability to express thoughts clearly accurately describes a communication disorder, as it directly relates to challenges in speech, language, or written communication.

The other choices do not align with the definition of a communication disorder. A pattern of aggressive behavior pertains more to behavioral issues rather than communication difficulties. Marked inappropriate social behavior also generally relates to social skills or interpersonal relations rather than the act of communicating itself. A decline in emotional stability can affect communication but is more indicative of emotional or psychological challenges rather than a specific communication disorder. Therefore, the correct choice highlights the core aspect of communication disorders—difficulty in effectively expressing thoughts.