Name and describe five ways in which plants can respond to their environment

The ways in which plants respond to the environment include; Thigmotropism which the plant's growth response to touch, Phototropism which is the direction growth of a plant towards light, geotropism which is response of a plant to gravity, hydrotropism which is the growth of plant towards a source of water ( mainly the roots) and lastly the response to daylength, which is a response of plants to different degree of light and darkness. 

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.