Jana always chooses the taller glass of grape juice when her older brother pours them both a glass of juice; even though there is the same amount of juice in each glass, she believes there is more juice in her glass. in this process, jana is demonstrating that she does not yet understand the concept of

I believe the answer is: concept of conservation
concept of conservation refers to the understanding that something would stay in the same quantity even though it is moved to different medium/appearances.
On average, understanding of concept of conservation would not be developed before we enter the age of 7.