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I'll go for C
D and E are wordy because they unnecessarily add personal pronoun in the second half of the sentence when relative pronoun "its" remove that element of wordiness
A and B are out because we need past perfect, something that happened prior to another event in the past.
An example with past perfect:
Those who saw Richard were surprised by how he (changed/had changed) in the past 18 years.
We have "in the past 18 years", an element of time. Had we not had that, simple past would have been fine. The above sentence, similar to the original one, means that Richard changed in the past 18 years, prior to those who saw him recently.