Fill in the blank with the most appropriate word.
I'm small. I wish I _____ small.
This sentence talks about the second conditional that is used to express imaginary and impossible situations. So, second conditional used forms of verbs in past tense. In case of second conditional, when ‘I/we wish’ is used to talk about a regret about a present or future event, then the expressions ‘I/We wish’ is followed by past simple. Hence, Option D is the correct answer here. Options A cannot be used as they use forms of verbs in present tense. Option B and C cannot be used as they would render the sentence grammatically incorrect.