The ‘bakanae’ (foolish seedling) disease of rice seedlings, was caused by : 

 Gibberella fujikuroi. 

The correct answer is Option (3)-  Gibberella fujikuroi.

The ‘bakanae’ (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid .

Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1 , GA2 , GA3 and so on.