Solution:

Prefixes are words that we increase another word in its beginning, thus forming a replacement word with a replacement meaning. Some prefixes are added so on create a word’s opposite or antonym.
Prefixes like il, in, un, non, and disserve this purpose. These prefixes signify the meaning ‘no’ or ‘not’.
The given sentence indicates that growing drug plants isn't legal in some parts of the planet. The one word for ‘not legal’ is ‘illegal’; formed by the addition of il to the foundation word ‘legal.’ Thus, option b is proved the proper answer.
Option a is wrong as un is usually used for words as in ‘unable’ or ‘unforgettable’. It denotes ‘not’.
Similarly, dis in option c is that the prefix used with words as – ‘disinterested’ or ‘disrespect’.
Non in option d is additionally wrong as nonprofessional, nonessential are the words that we commonly use therewith prefix.