I can see why, to a non-native speaker, the words should and would might look almost the same in this situation. However, to a native English speaker the answer, by instinct, is obvious: 2) should - it cannot be would. As to why it is "should," the answer is more in the realm of idiom and traditional usage (evolved over hundreds of years), than grammatical theory.
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dimsumexpressShould she call, tell her I am not home = If she happens to call, tell her I am not home. [...] There is no conditional element of any kind in this sentence.Well, it is a conditional sentence.
fivejedjonWell, it is a conditional sentence.Correct me if I am wrong. Although the equation suggested its conditional tone, "should" is never described and considered as an conditional element in the traditional grammar; is it? . Maybe " Had she called..." Or, " If she calls......