Choice "A" is correct. A firewall is an "electronic device" (a firewall may actually be both hardware and software and not just hardware) that prevents unauthorized users from gaining access to network resources. A firewall isolates a private network of some type from a public network (or a network segment from the main network). It also maintains a (controlled) connection between those two networks.
Choice "c" is incorrect. A query program has nothing to do with connecting networks or with separating or isolating a network segment from the main network. A query program is a program that allows a user to obtain information from a database or other data source.
Choice "d" is incorrect. An image browser is a program that displays a stored graphical image. It has nothing to do with connecting networks or with separating or isolating a network segment from the main network. An image browser is used to display information from a database or other data source.
Choice "b" is incorrect. In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language being used. For example, some people have seen things like (IF…THEN) in some basic programming languages (FORTRAN, COBOL, Visual Basic, and many others). Both IF and THEN are keywords, and they cannot be used in that language out of their specified context. Alternatively, in a search, a keyword is a word that is used to find information somewhere that contains that word. Either way, however, a keyword has nothing to do with connecting networks or with separating or isolating a network segment from the main network.