Correct Answer: Activated carbon

Solution : The correct option is Activated carbon.

Powdered activated carbon is the suggested remedy for tastes, and odours that are modest and sporadic. By adding powdered activated carbon to water, or using granular activated carbon in the water filter, bad tastes, and unpleasant odours can be eradicated. The way activated carbon functions is by a process called adsorption, in which the component responsible for the odour is trapped inside the activated carbon, and kept without affecting the size of the substance doing the adsorption.

Correct Answer:  can be helped

Solution : There is an error in the option 4.
We need to use "Can be helpful" instead of " can be helped".

Help is a verb however helpful is adjective. 

Help means by offering one's services or resources, one can make anything easier or achievable for (someone). Helpful means willing to help, or useful:

Hence the correct sentence is 

In order to give up the bad habit of drinking, use of medicine or  similar other method can be helpful.

Correct Answer:

It was a bad idea and it was not risk-free

Solution :

We need ‘and’. Hence, “It was a bad idea and it was not risk-free” is the best choice

Correct Answer: Bad luck

Solution : The correct option is the third option.

The idiom, bad iron means to have bad luck or be unfortunate.

Correct Answer: Ameliorate

Solution : The correct choice is the fourth option.

Explanation:
Ameliorate fits this requirement, as it means to make something bad or unsatisfactory better. It is a synonym for improve and precisely conveys the meaning of enhancing a situation or condition.

The meanings of the other options are as follows:

• Judge: This word means to form an opinion or decide upon after careful consideration.
• Malign: This word means to speak harmful untruths about someone.
• Castigate: This word means to reprimand or criticise severely.

Therefore, the correct answer is ameliorate.