To answer this question, the user needs to know the definition and purpose of beta testing.

Beta testing is a type of software testing that involves allowing real users to test a product before it is released.

Now, let's go through each option and explain why it is right or wrong:

A. Performed by customers at their own site: This option is correct. Beta testing is often performed by customers at their own site, as they are the ones who will be using the software in the real world and can provide valuable feedback to the developers.

B. Performed by customers at their software developer’s site: This option is incorrect. While customers may be invited to the software developer's site for testing, beta testing is generally performed at the customer's own site.

C. Performed by an independent test team: This option is incorrect. Beta testing is usually performed by real users, not an independent test team.

D. Useful to test bespoke software: This option is correct. Beta testing can be useful for testing bespoke software, as it allows the developers to get feedback from real users before releasing the product.

E. Performed as early as possible in the lifecycle: This option is incorrect. Beta testing is usually performed after the software has gone through alpha testing and is close to release.

Therefore, the correct answer is: A. Performed by customers at their own site.