Concept:
The adjective maiden means first, initial, or inaugural. The expression maiden speech refers to the first formal speech delivered by a person, especially in a legislative body, public office, or important gathering.
Step 1: Understand the phrase.
\[
\text{Maiden speech}
\]
means the first official speech made by someone.
Step 2: Apply the meaning to the sentence.
The speaker was nervous because it was his first public speech.
Step 3: Check the options.
• Last speech -- incorrect.
• Unprepared speech -- not implied.
• First speech -- correct.
• Unintelligible speech -- unrelated.
Step 4: Choose the correct answer.
\[
\boxed{\text{First speech}}
\]