Work it out by eliminating the obviously incorrect answers:
'Synthetic' means man-made (usually to imitate a natural products, such as 'synthetic rubber'), so that's got rid of answer (b) straight away.
A simple bit of environmental awareness shold tell you that plastics (which are synthetic materials) often aren't good for the environment. (Think of the campaigns against the use of plastic carrier bags). So that's answer (a) thrown out of the window.
Which leaves you with . . . ?