Select the most appropriate synonym to replace the underlined word in the given sentence.
No altruistic act is truly sincere.

The correct answer is philanthropic.

Step-by-step Explanation:

1. Analyze the underlined word:
The word altruistic is an adjective that describes behavior showing a selfless concern for the well-being of others; it means unselfish, caring, or benevolent.

2. Evaluate the given options:
• phenomenal: extraordinary, outstanding, or remarkable. This does not match the meaning of selfless care.
• phonotypical: relating to the physical or observable characteristics of an organism (typically spelled phenotypic). This is unrelated to human behavior or kindness.
• philanthropic: seeking to promote the welfare of others, especially by donating money, time, or resources to good causes; benevolent and unselfish. This is a direct synonym for altruistic.
• phantasmal: like a phantom, illusory, or unreal. This does not fit the context of the sentence.

3. Conclusion:
Replacing "altruistic" with "philanthropic" maintains the original meaning of the sentence: "No philanthropic act is truly sincere." Therefore, philanthropic is the most appropriate synonym.