Our plane ... because of weather conditions yesterday.

The sentence describes a past event ('yesterday') and a state that resulted from that event. The passive voice is appropriate here because the plane is acted upon. The past tense is required due to 'yesterday'. Therefore, 'was delayed' is the correct form. The options 'is delayed' and 'delayed' are incorrect because they do not fit the past tense context. 'was delaying' is incorrect because it implies the plane was actively causing a delay, which is not the intended meaning.