The machine is difficult to build …… easy to maintain. A. but B. and C. for D. if

The correct answer is but.

The word “but” is used to introduce a contrast or contradiction. In this case, the machine is difficult to build but easy to maintain. This means that the machine is not easy to create, but it is not difficult to keep in good condition.

The other options are not as accurate. The word “and” is used to join two things together. In this case, it would not make sense to say that the machine is difficult to build and easy to maintain. The word “for” is used to introduce a reason or purpose. In this case, it would not make sense to say that the machine is difficult to build for easy maintenance. The word “if” is used to introduce a condition or possibility. In this case, it would not make sense to say that the machine is difficult to build if easy to maintain.

In conclusion, the correct answer is but.