The correct option is
C effective
Option C: 'Effective' means 'successful in producing a desired result'.
Here the medicines are successful in producing the desired result 'curing a cold'. Hence this word forms a complete meaningful sentence and option C is correct.
Option A: 'Proper' means 'suitable, appropriate'.
This word is not as suitable as option C.
Option B: 'Real' means 'not artificial, genuine'.
Option D: 'Capable' means 'able to achieve efficiently'.
Option E: 'Powerful' means 'having great power or strength'.
These words do not form a complete meaningful sentence.
Hence options A, B, D and E are wrong.