I want to pass a variable with the block code like of JSON type in magento,
{{block type=\"multibanners/multibanners\" category_id=\"9\" name=\"multibanners\
I think the problem here stems from the block type you are calling. When you define a type, you're telling Magento to load that model and pass it the appropriate data - which then only exposes the functions defined on that specific model.
A better solution may be to reference the core block type "core/template" which exposes the ->getData() method, and then load the "multibanners/multibanners" model to work with and output the data.
{{block type="core/template" category_id="9" name="multibanners" alias="multibanners" template="multibanners/multibanners.phtml"}}
I'm not sure what the proper syntax is to load 'multibanners', but in the multibanners.phtml would be something like this:
getData('category_id');
$multibanner = Mage::getModel('multibanners/multibanners')->load($catId);
/**
** Generate some output here.
*/
?>