What term describes someone who does not readily reveal their thoughts or feelings?

The term that best describes someone who does not readily reveal their thoughts or feelings is "reticent." This word conveys a sense of reservedness or avoidance of speaking one's mind, typically implying a deliberate choice to keep personal emotions or opinions hidden.

In this context, someone who is reticent may be hesitant to share their thoughts, opting for silence or vague responses instead. This quality of being reserved is often associated with various interpersonal dynamics, where an individual may prefer to remain private or discreet about their inner world.

The other terms, while having their meanings, do not align with the description given in the question. "Confounded" generally suggests confusion or bewilderment, "ostentatious" refers to an excessive display of wealth or knowledge, and "dismal" describes something that is gloomy or depressing. Therefore, none of these words encapsulate the idea of someone who is reserved or quiet about their feelings in the way that "reticent" does.