Which genre includes pieces with descriptive titles meant to convey their themes?

The correct genre that includes pieces with descriptive titles intended to convey their themes is indeed character pieces. Character pieces are often short instrumental compositions that express a specific mood or idea, with titles that hint at their emotional or narrative content. These pieces are designed to evoke imagery or a particular character, making them distinct in their descriptive nature.

In contrast, a symphony typically refers to a larger orchestral work that may not have descriptive titles and is more focused on the structure and development of themes over multiple movements. Opera encompasses vocal works that tell a story through music and drama but relies on lyrics and acting to convey themes rather than relying solely on descriptive titles. A sonata, while being a significant form in classical music, often follows a specific structure and may not emphasize descriptive thematic titles in the same way that character pieces do.

Thus, the nature of character pieces makes them particularly notable for their use of evocative titles to convey themes and emotions, setting them apart from the broader categories of symphonies, operas, and sonatas.