The line "a tree whose hungry mouth is prest" is an example of iambic
a. trimeter.
b. pentameter.
c. tetrameter.
d. dimeter.

Since the only way to find out if a line is written in a trimeter, or pentameter, etc. is to count the number of syllables, and then divide that number by two to get the meter, that is exactly what we are going to do here.
This line has 8 syllables: (a tree whose hung- ry mouth is prest), we should divide it by 2, which equals 4. 
So, the correct answer is that this line is an example of iambic tetrameter. Tetra means 4.