The manual holds that women mustn't choose a programme themselves, but instead wait to be assigned to a module.

You use a comma before "but" if it introduces an independent complete clause.

As there's no independent clause after "but" in the sentence presented above, it doesn't need a comma before "but".

The manual holds that women mustn't choose a programme themselves, but instead wait to be assigned to a module.

You use a comma before "but" if it introduces an independent complete clause.

As there's no independent clause after "but" in the sentence presented above, it doesn't need a comma before "but".

The manual holds that women mustn't choose a programme themselves, but instead wait to be assigned to a module.

You use a comma before "but" if it introduces an independent complete clause.

As there's no independent clause in the sentence presented above, it doesn't need a comma before "but".

The manual holds that women mustn't choose a programme themselves, but instead wait to be assigned to a module.

You use a comma before "but" if it introduces an independent complete clause.

As there's no independent clause after "but" in the sentence presented above, it doesn't need a comma before "but".