How can we retain dropdown list value, when we select the option?
You can set the default values as follows,

$form['choice'] = array(
    '#type' =&gt; 'select',
    '#options' =&gt; $options,
    '#title' =&gt; t('Select your layout'),
    '#default_value' =&gt; variable_get('choice', $form_state['choice']['type']
)
);

provided you get the submitted values in the variable $form_state['choice']['type'] post submit action.
And the variable name should match the form field name "choice"

Hope this works for you!