The key to exercise solutions in chapter 2A - Are you tidy or untidy? for the textbook English File Elementary - Student's Book with authors Christina Latham-Koenig, Clive Oxenden and Jerry Lambert from Oxford University Press
Question
Complete the chart. Why is it a pen but an umbrella?
Answer
(two) umbrellas
a watch
a diary
Because it begins with a vowel sound, not a consonant sound.