Some species of blue-green algae have the ability to convert atmospheric nitrogen into a form that can be absorbed readily by crop plants. This is the feature that helps promote them as bio-fertilizers.

These blue-green algae, also known as cyanobacteria, are able to carry out nitrogen fixation, which involves the conversion of atmospheric nitrogen into ammonia or other forms of nitrogen that can be used by plants. This process is carried out by specialized cells called heterocysts, which are capable of creating a microenvironment that is conducive to nitrogen fixation.

By introducing these nitrogen-fixing cyanobacteria into the soil, farmers can improve the fertility of their crops without the need for synthetic fertilizers. This can be especially beneficial for organic farming, where the use of synthetic fertilizers is not allowed.

Therefore, the correct answer is (c) They have the mechanism to convert atmospheric nitrogen into a form that crop can absorb readily.