Step 1: Understanding the phrase.
The idiom ‘hold good’ means to remain valid, applicable, or effective.
Step 2: Analysis of options.
- Happy: Incorrect, unrelated to the phrase.
- Thoughtful: Incorrect, unrelated.
- Effective: Correct, as ‘hold good’ means to remain effective or valid.
- (Hopeful: Incorrect.
Step 3: Conclusion.
Hence, the correct answer is effective.