Short Answer 
The analogy compares components in two sets: a letter and a word, and a note and a song. Both relationships illustrate how smaller units are fundamental in forming larger structures, concluding that the correct answer for the analogy is ‘song’.
Identify the Components
In the analogy, we compare two sets of components. The first set involves a letter and a word, while the second set includes a note and a song. Each component represents a smaller unit that contributes to the formation of something larger and more complex.
Understand the Relationships
Both relationships depict how smaller units come together to create a whole. A letter is an essential part of a word, just as a note is fundamental in constructing a song. This reinforces the idea that each smaller unit is necessary for the complete form of the larger unit.
Conclude with the Correct Completion
By analyzing the relationships established in the analogy, we derive that the correct completion for ‘Letter is to word as note is to _______’ is ‘song’. Therefore, the answer is Option B, highlighting the connection between these elements.