Final answer:
Earl Scruggs and Lester Flatt joined Bill Monroe's band in 1945 and had a major impact on the band's sound.
Step-by-step explanation:
The correct answer is option a. Earl Scruggs and Lester Flatt.
In 1945, Earl Scruggs and Lester Flatt joined Bill Monroe's band, the Blue Grass Boys, and had a major impact on the band's sound. Earl Scruggs revolutionized banjo playing with his innovative three-finger picking style, while Lester Flatt's smooth vocals and rhythm guitar skills contributed to the band's distinctive sound.
This collaboration with Scruggs and Flatt helped define the sound of bluegrass music and solidified Bill Monroe's role as the father of bluegrass.