A fuse, marked 250v, 650ma can be used in circuits with an "open-fuse" supply voltage of:

The designation on the fuse, labeled as 250V and 650mA, indicates the maximum voltage and current the fuse can safely handle. The voltage rating of a fuse is crucial, as it specifies the highest voltage the fuse can interrupt without the risk of arcing or explosion if the fuse blows.

In this case, the "250V" rating indicates that this fuse is suitable for use in circuits where the supply voltage does not exceed 250 volts. Using the fuse in a circuit with a voltage higher than this limit could result in the fuse failing to operate properly and could potentially create a safety hazard. Therefore, the appropriate voltage for which this fuse can be used, while adhering to safety standards and ensuring effective operation, is indeed a maximum of 250 volts.

The other choices suggest voltages that either exceed the rated voltage of the fuse, which is not safe or permissible, or are operating below the standard use voltage for many applications. The most suitable answer is the maximum voltage rating, which remains at 250 volts.