💡 Detailed Explanation
The correct choice is 'よ' (forming 'よければ' - if it is good). The conditional '〜ば' form of the irregular I-adjective 'いい/よい' (good) is created by changing 'よい' to 'よければ' (yo + kereba). Sentence translation: 'If the weather is good tomorrow, let's go on a picnic.' 'いいければ', 'よかっければ' are grammatically invalid.