One way a parent can tell that his or her adolescent is beginning to develop formal operational thinking is by the appearance of thinking.

Abstract thinking

Explanation:

 The abstract thinking is refers to the thinking ability of the person about the ideas and the principles and it is basically helps in developing the various types of experiences and the objects.

The main objective of the abstract thinking is that it developing the problem solving ability and also improve in our formal operational thinking ability. This concept is also related to the symbolic thinking.  

 Therefore, Abstract thinking is the correct answer.

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