彼は作家としてはまだまだ__________表現が多いが、人の心を打つストーリーが書ける。
• ふつつかな• 不得意な• つたない• つれない
English: "As a writer, he still has a lot of poor phrasing, but he can write stories which strike people's hearts"
ふつつか（な）- inexperienced
A way to say someone is inexperienced, but it's only used to refer to oneself in a humble situation. (Using it to refer to someone else is rude.)
e.g ふつつかながら、僕でよければご協力しますよ！ - I may be inexperienced, but I'll be happy to help!
不得意（な）- one's weak point
e.g 彼は英語が好きだが数学は不得意だ。- While he likes English, he is weak in mathematics.
つれない - cold, indifferent
e.g そんなことを言うなんて彼はつれない。- It is heartless of him to say so.
つたない - unskilled, poor-quality
e.g つたない文章で恐縮ですが、ご覧いただけますと幸いです。
The hint in this question is in the が followed by a positive phrase. We know the first clause must be something negative and only つたない makes sense in the context of 表現.
