Step 1: Understanding the Question:
The task is to find the phrase that matches the meaning of "leitmotif".
Step 2: Key Formula or Approach:
Trace the word back to its origin in music, where it names a phrase that keeps returning, then look for the option that keeps that sense of repetition.
Step 3: Detailed Explanation:
In opera, a leitmotif is a short musical idea tied to a character or event that plays again whenever that character or event appears. Writers borrowed the word to describe any theme, image or idea that reappears through a book, film or speech.
"Spirit of the age" describes the general mood of a time period, not something that repeats within one work.
"Main design" points to overall structure or plan, with no sense of repetition.
"Chief objective" names a goal, unrelated to a returning idea.
Only "recurring theme" keeps the essential sense of something coming back again and again, which is what a leitmotif does.
Step 4: Final Answer:
The correct synonym for "leitmotif" is "recurring theme", so the answer is option (a).