The correct answer is option 'C' - costly and effective medicines.

Explanation:
Prescriptions are given by doctors or specialists to provide specific instructions for medical treatment. Prescriptions can include various types of treatments depending on the condition of the patient and the desired outcome. In this case, the specialist prescribed costly and effective medicines.

1. Cheap and effective medicines:
- Cheap medicines are generally affordable and can be suitable for mild conditions or common ailments that do not require strong medications.
- These medicines are widely available and can be purchased over the counter without a prescription.
- However, they may not be as potent or effective as more expensive medications for serious or chronic illnesses.

2. Herbal treatment:
- Herbal treatment involves the use of plants or plant extracts to treat various ailments.
- It is often considered a natural and alternative form of medicine.
- Herbal treatments can be effective for certain conditions, but their efficacy varies and may not be scientifically proven for all illnesses.
- They are generally considered safer and have fewer side effects compared to synthetic medications.

3. Costly and effective medicines:
- Costly medicines are usually more expensive due to their production, research, and development costs.
- They are often prescribed for serious or chronic conditions that require stronger medications with proven efficacy.
- These medicines typically undergo rigorous testing and clinical trials to ensure their safety and effectiveness.
- They may have specific dosages and instructions to be followed to achieve the desired results.

4. Hilly environment:
- The mention of a hilly environment in the options seems unrelated to the prescription itself.
- It is not a typical form of medical treatment and does not directly address the patient's condition or illness.
- It is possible that the hilly environment option was included as a distractor to confuse the reader.

In conclusion, the specialist prescribed costly and effective medicines because they are likely to provide the best results for the patient's condition. The other options (cheap medicines, herbal treatment, and hilly environment) are not as suitable or relevant in this context.