Step 1: Recall the transport form of fixed nitrogen in legumes.
In tropical legumes such as soybean, the fixed nitrogen is transported from the root nodules mainly in the form of
\[
\boxed{\text{Ureides}}
\]
such as allantoin and allantoic acid.
Step 2: Choose the correct option.
Hence, soybean nodules export the fixed nitrogen as
\[
\boxed{\text{Ureides}.}
\]
Therefore, the correct option is \(\boxed{(C)}\).