Mathematical symbols representing probability events

A number of multiple choice questions which I am working on relate to mathematical symbols which represent probability events…in order to tackle the questions, I need to be clear what is/ is not a valid mathematical symbol…. Are a, b, c, d (see below) valid mathematical symbols which can be used to represent a probability event? I think a, b and c are valid symbols, but am not sure about d - can a single letter be used to represent a probability event? I would appreciate any clarification on these symbols i.e. confirmation they are/ are not valid probability symbols. Thanks in advance.

a) S ? R (intersection)
b) S ? R (union)
c) ~S (complement)
d) S

NB
S stands for swimming, R stands for running

\(\displaystyle S\cap R\) means the probability of Standing AND Running. One can not do both at the same time, so this would not be valid.

\(\displaystyle S\cup R\) means the probability of standing OR running. This is valid because you do one or the other.

~\(\displaystyle S\) means the probability of NOT standing. The complement of S is everything not in S.

galactus said:

\(\displaystyle S\cap R\) means the probability of Standing AND Running. One can not do both at the same time, so this would not be valid.

\(\displaystyle S\cup R\) means the probability of standing OR running. This is valid because you do one or the other.

~\(\displaystyle S\) means the probability of NOT standing. The complement of S is everything not in S.

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Thanks very much. So does S on its own simply represent the probability of standing?