In the context of string theory, the "extra dimensions" proposed in various models suggest that:
a) There are additional spatial dimensions beyond the familiar three dimensions of space and one dimension of time, which are compactified and not directly observable.
b) The universe has only three spatial dimensions, and time is the only additional dimension.
c) Extra dimensions are purely theoretical and do not have any physical significance.
d) The extra dimensions are large and observable in everyday experiences.