A four-bar chain is a mechanism that consists of four links connected by four joints. The joints can be classified into two types: turning pairs and sliding pairs. A turning pair allows rotation between two links, while a sliding pair allows linear motion between two links.

In a four-bar chain, there are several combinations of joint types that can be present. The question asks for the correct combination of joint types in a four-bar chain.

Understanding the given options:
Let's analyze each option given in the question and understand the joint types present in each case:

a) All turning pairs:
This means that all four joints in the four-bar chain are turning pairs. In other words, each joint allows rotation between two links.

b) One turning pair and the others are sliding pairs:
In this case, only one joint in the four-bar chain is a turning pair, while the remaining three joints are sliding pairs. This means that one joint allows rotation, while the other three joints allow linear motion.

c) One sliding pair and the others are turning pairs:
In this case, only one joint in the four-bar chain is a sliding pair, while the remaining three joints are turning pairs. This means that one joint allows linear motion, while the other three joints allow rotation.

d) All sliding pairs:
This means that all four joints in the four-bar chain are sliding pairs. In other words, each joint allows linear motion between two links.

Identifying the correct option:
From our understanding of the different joint types and the given options, we can conclude that the correct answer is option 'A' - all turning pairs. In a four-bar chain, all the joints are usually turning pairs, allowing rotation between the links. This configuration provides a wide range of motion and is commonly used in various mechanical systems.

Conclusion:
A four-bar chain generally consists of all turning pairs, allowing rotation between the links. This configuration provides flexibility and is commonly used in mechanical engineering applications.