The most appropriate reason for the author to be spared many a mishap is that

The correct option is: he hardly utters or writes a thoughtless word.

To understand why this is the most appropriate reason, let us analyze the connection between careful communication and avoiding mishaps:

1. The Power of Deliberate Speech and Writing: Many personal, social, and professional conflicts or "mishaps" stem from hasty, ill-considered remarks or written statements. When a person speaks or writes without thinking, they are far more likely to cause misunderstandings, offend others, or commit to actions they cannot fulfill.

2. Prevention of Conflict: By ensuring that one "hardly utters or writes a thoughtless word," an individual exercises a high degree of mindfulness and self-restraint. Every word is weighed for its truth, relevance, and impact. This habit prevents the escalation of arguments and avoids the pitfalls of misinformation or emotional outbursts.

3. A Shield Against Regret: Deliberation in communication acts as a protective shield. When words are chosen with care, there are fewer errors to correct, fewer apologies to make, and fewer controversies to manage, thereby naturally sparing the person from many unnecessary mishaps in life.