These medicines are _______ for curing cold.

The most appropriate word to fill in the blank is effective.

The other options are not as accurate. Proper suggests that the medicines are suitable for curing a cold, but this does not necessarily mean that they are effective. Real suggests that the medicines are genuine, but this does not necessarily mean that they are effective. Capable suggests that the medicines are able to cure a cold, but this does not necessarily mean that they are effective in practice. Effective is the best option because it suggests that the medicines have been shown to be successful in curing colds.