Step 1: Observe the tense of the two actions mentioned in the sentence. The first clause is in the simple past tense, while the action of reading took place before the action of understanding.
Step 2: When one past action occurs earlier than another past action, the earlier action should be expressed using the past perfect tense. Therefore, the correct form is “had read.”
Step 3: The correct sequence of tenses is:
\[
\text{understood (later)} + \text{had read (earlier)}
\]
Step 4: Hence, the option that follows this correct tense sequence is (d).