(C) Change in amount of water in cells
When chhui-mui is touched, plant cells change shape by changing the amount of water in them, causing swelling or shrinking, which results in the leaves folding.
Source: Chapter 6, Section 6.2.1 – Immediate Response to Stimulus
The textbook explicitly states: "plant cells change shape by changing the amount of water in them, resulting in swelling or shrinking." This is not a hormonal response (hormones cause slow, growth-related movements like bending towards light). The movement of the sensitive plant is rapid and growth-independent — remember this distinction for both MCQs and short-answer questions.