The correct answer to this "security". It is a major factor in deciding if a centralized or decentralized environment is more appropriate.

Example: In a centralized computing environment, you have a central server and workstations (often "dumb terminals") access applications, data, and everything else from that central servers. Therefore, the vast majority of your security resides on a centrally managed server. In a decentralized (or distributed) environment, you have a collection of PC's each with their own operating systems to maintain, their own software to maintain, local data storage requiring protection and backup. You may also have PDA's and "smart phones", data watches, USB devices of all types able to store data... the list gets longer all the time.

It is entirely possible to reach a reasonable and acceptable level of security in a distributed environment. But doing so is significantly more difficult, requiring more effort, more money, and more time.

The other answers are not correct because:

scalability - A distributed computing environment is almost infinitely scalable. Much more so than a centralized environment. This is therefore a bad answer.

heterogeneity - Having products and systems from multiple vendors in a distributed environment is significantly easier than in a centralized environment. This would not be a "challenge of distributed computing solutions" and so is not a good answer.

usability - This is potentially a challenge in either environment, but whether or not this is a problem has very little to do with whether it is a centralized or distributed environment. Therefore, this would not be a good answer.