Thigmotropism is the response of the plant to

Match the following:
Column-$A$ | Column-$B$
$(1)$ Auxin | $(I)$ Ripening and maturity of fruits
$(2)$ Gibberellin | $(II)$ Differentiation of xylem elements
$(3)$ Cytokinin | $(III)$ Prevention of genetic and physiological dwarfism
$(4)$ Ethylene | $(IV)$ Found from tumour tissue of tobacco

The pigment involved in photomorphogenetic movements is

$Cuscuta$ is a

During experimentation, why is it difficult to attribute any observed effect to a single hormone?

Which one of the following pairs of plant hormones inhibits flowering in some plants?