Iodine is necessary for the synthesis of which hormone?

$(a)$ Name the diseases by which a person is likely to suffer due to the deficiency of:
$(i)$ Iodine
$(ii)$ Insulin
$(b)$ How are the timing and amount of hormone secretion regulated in the human system? Explain with an example.

The response of plants to stimuli is limited because $...$

In what form is the response to a stimulus generally observed in body parts?

What is a tropic movement? Explain with an example.

Which of the following statements are true?
$(i)$ Sudden action in response to something in the environment is called reflex action.
$(ii)$ Sensory neurons carry signals from spinal cord to muscles.
$(iii)$ Motor neurons carry signals from receptors to spinal cord.
$(iv)$ The path through which signals are transmitted from a receptor to a muscle or a gland is called reflex arc.