The machine is difficult to build ...... easy to maintain.

Solution (By Examveda Team)

The machine is difficult to build but easy to maintain.
In this context, "but" is used to contrast two contrasting statements. It connects the difficulty in building the machine with the ease of maintaining it, emphasizing the difference between the two tasks. Therefore, "but" is the correct choice to complete the sentence, conveying the intended contrast between building and maintaining the machine.