Telephone services are an essential means of communication in our daily lives. Within a country, there are several types of telephone services available for use. The correct answer to this question is option 'D', which states that both (a) PCO and (b) STD services can be used within the country. Let's explore these services in detail.

1. PCO (Public Call Office) Services:
- PCO services are commonly found in public places like bus stations, railway stations, airports, and marketplaces.
- These services allow individuals to make local calls within the same city or town.
- PCOs are operated by service providers who provide public telephones for people to make calls by inserting coins or using prepaid cards.

2. STD (Subscriber Trunk Dialing) Services:
- STD services enable individuals to make long-distance calls within the country.
- These services are used for making calls to different cities or towns within the same country.
- STD calls are charged at different rates depending on the distance and duration of the call.
- To make an STD call, one needs to dial the STD code of the particular city or town followed by the recipient's telephone number.

3. ISD (International Subscriber Dialing) Services:
- ISD services allow individuals to make international calls to other countries.
- These services are used for making calls to people residing in other countries.
- ISD calls are charged at higher rates compared to local or STD calls.
- To make an ISD call, one needs to dial the international access code of the respective country, followed by the country code, area code, and the recipient's telephone number.

In conclusion, within a country, individuals can use both PCO and STD services for making local and long-distance calls, respectively. ISD services, on the other hand, are used to make international calls. These telephone services play a crucial role in connecting people and facilitating effective communication.