The correct answer is: a) Soft and lyrical.
Folk songs are typically passed down orally from generation to generation, and as such, they often have a simple, repetitive melody that is easy to remember and sing. The vocal style in folk songs is often soft and lyrical, with a focus on the melody rather than the harmony. This is in contrast to many other genres of music, which often have a more complex melody and harmony.
Option b) is incorrect because folk songs are not typically loud and energetic. They are often slow and simple, with a focus on the melody.
Option c) is incorrect because folk songs are not typically highly ornamented with melismas. Melisma is a technique in singing where a single syllable is sung on multiple notes. This is not typically found in folk songs.
Option d) is incorrect because folk songs are not typically based on strict harmonic rules. They often have a simple, repetitive melody that is easy to remember and sing.