Answer: B) Daffodils.
Step-by-step explanation: In the given poem by William Wordsworth, the bolded line "They stretched in never-ending line" is referring to the daffodils, we can know that because in the first stanza the speaker is describing a vale with golden daffodils, and in the second stanza the speaker compares those daffodils with stars (Continuous as the stars that shine , And twinkle on the milky way) and says that they (the daffodils) stretched in never-ending line along the margin of a bay.