Option 1 -> "agreeable to" means willing to accept something, which doesn't express causation.
Option 2 -> "on behalf of" means representing someone/something, which is illogical with a weather forecast.
Option 3 -> "on account of" means because of or due to, which correctly shows the rain forecast as the reason for postponement.
Option 4 -> "in favour of" means in support of something, which doesn't fit the context of explaining a reason.
Hence, Option 3: on account of -> This phrase is used to indicate the cause or reason for an action. The meeting was postponed because of (on account of) the heavy rain forecast, making this the grammatically correct and contextually appropriate choice -> correct