What is the poetic form of this excerpt from Robert Frost's "Mending Wall"?

"Mending Wall" is one of Robert Frost's most famous poems. When you are asked about the form of something, you need to start with the very basics. At root, the form is "poetry." Delving more deeply, we consider what type of poetic form Frost has chosen. In this instance, we can see that the lines do not rhyme with each other, so it isn't a traditional poetic form like a sonnet or a ballad. However, at the same time, if you sound out the lines, you can hear that they fall into a distinct rhythm and pattern. This regular meter—known as iambic pentameter, because there are five stressed beats, or feet, per line—means that this is not a free verse poem. Although it does not rhyme, these metrical verse features mean that we refer to it as blank verse.

Explanation:

the answer is b. the tomb is underground and dark, and its opening is covered by a heavy grate.