Hi,
a. is in alphabetical order because first, they all begin with 'tr' so we look at the third letter. It goes a, e, i, o (which is how it goes on the alphabet)
b. doesn't, because is should be: pearl, peck, pedal, peek.
c. doesn't as well, because it should go: waist, wall, warn, wave
d. doesn't go either because it should be: lack, lamb, lava, and then law.
So it's answer a.
