What is overflow?

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When a binary value is too small to be stored in the bits available
When a binary value is too large to be stored in the bits available
When a denary value is too large to be stored in the bits available
None of the above

what is "denary"? You mean decimal?

in any case, binary or decimal, overflow means too large (positive or negative)
underflow occurs when a floating-point value is too close to zero, so that the negative exponent cannot be stored in the bits available