Hormone which

a. is gaseous in nature: ethylene

Ethylene is a small hydrocarbon gas which is an ageing hormone in different plants. It causes ripening of fruits as well as has growth inhibitory functions.

b. is responsible for phototropism: auxin

Presence of auxins is responsible for plant response to light intensities. Plant cells further away from the light has more auxin thus grows faster aiding bending of the stem towards the light source.

c. induces femaleness in flowers of cucumber: ethylene

Ethylene induces flowering so can be used to increase the number of female flowers in the cucumber plants. It selectively induces flowering of female plants suggesting a link between ethylene signalling and primordial anther specific DNA damage in female cucumber flowers.

d. is used for killing weeds (dicots): auxins

Auxin growth regulator herbicides are used for controlling annual, simple perennial etc. They are selective and is widely used to kill dicotyledonous weeds whereas not affecting mature monocotyledonous plants.

e. induces flowering in long day plants : Gibberellin

Usually after gibberellin is applied to long-dayplants, stem elongation occurs before flower primordia are formed.