They're both correct, but they're grammatically different. It isn't simply a case of adding or omitting a preposition if you feel like it. If 'lacking' is a verb, you don't use 'in'; if 'lacking' is an adjective, you do. The sauce was lacking flavour = The sauce lacked flavour. Here, 'lack' here is a verb, and 'flavour' is the direct object. The sauce was lacking in flavour. Here, 'lacking' is a predicative adjective which requires a particular construction: be + lacking + in + indirect object I hope that helps.

They're both correct! As another commenter already said, the preposition is correct but not nessecary.

Both are correct. No preposition is required but you can put it there if you like.

Both are correct. No preposition is required but you can put it there if you like.