Final answer:
An Italian meringue is correctly whipped when it forms stiff peaks, which means the mixture stands up straight when the beaters are lifted.
Step-by-step explanation:
To know when an Italian meringue has been whipped long enough, look for the point at which it forms stiff peaks. This occurs when the meringue stands up straight when the beaters are lifted from the mixture. The meringue should also be glossy and smooth. If it starts to turn yellow, it may be overbeaten, which is not desirable. A properly whipped meringue should neither become runny nor should it have a strong sweet smell specific to whipping; these are not indicators of its readiness. Therefore, the correct answer to the multiple-choice question is (a) It forms stiff peaks.