が

午後(ごご)からは 強(つよ)い 雨(あめ) が 降(ふ)る みたいです。

The correct answer is "が". This sentence uses the pattern "~みたいです" (it seems like ~) to express a conjecture. The content of the conjecture is "強(つよ)い雨(あめ)が降(ふ)る" (strong rain will fall). Within this embedded clause, "強(つよ)い雨(あめ)" (strong rain) is the subject of the verb "降(ふ)る" (to fall), and subjects are marked by the particle "が". Therefore, "強(つよ)い雨(あめ)が降(ふ)るみたいです" is grammatically correct. "強(つよ)い" modifies "雨(あめ)", and "雨(あめ)" is the noun subject, both already present. "降(ふ)る" is the verb and comes after the subject and particle.

正解は「が」です。「〜みたいです」は推量を表す表現です。推量の内容は「強い雨が降る」で、「強い雨」が動詞「降る」の主語であるため、主格助詞の「が」が必要です。「強い」と「雨」はすでに文中にあり、「降る」は主語の後ろにくる動詞のため不適当です。