You have a Web Part that causes an error.

You need to ensure that you can view the trace history in the Trace.axd file.

What should you configure in the web.config file?

A.
In the <system.web> element, add the following line of code:
<trace enabled=”true” localOnly=”true” pageOutput=”false” />

B.
In the <SafeMode> element, configure the following attributes:
CallStack=”true”
AllowPageLevelTrace=”true”

C.
In the <SafeMode> element, configure the following attributes:
CallStack=”false”
AllowPageLevelTrace=”true”

D.
In the <system.web> element, configure the following element:
<trace enabled=”false” localOnly=”true” pageOutput=”true” />
In the <SafeMode> element, configure the following attribute:
AllowPageLevelTrace=”true”

Explanation:
MNEMONIC RULE: “Trace.axd = trace enabled=”true””

trace Element (ASP.NET Settings Schema)
http://msdn.microsoft.com/en-us/library/6915t83k.aspx