Concept:
In indirect speech, tense and time expressions change.
Step 1: Change pronoun.
\[
I \rightarrow he
\]
Step 2: Change tense.
\[
am \rightarrow was \quad ({present} \rightarrow {past})
\]
Step 3: Change time expression.
\[
now \rightarrow then
\]
Step 4: Form sentence.
\[
{Clinton said that he was very busy then.
\]
Hence, the correct answer is option (D).