newsupernova/app/code/Smartwave/Porto/Model/Config/Settings/Newsletter/Enable.php

<?php
namespace Smartwave\Porto\Model\Config\Settings\Newsletter;

class Enable implements \Magento\Framework\Option\ArrayInterface
{
    public function toOptionArray()
    {
        return [
            ['value' => '0', 'label' => __('Disable')], 
            ['value' => '1', 'label' => __('Enable on Only Homepage')], 
            ['value' => '2', 'label' => __('Enable on All Pages')]
        ];
    }

    public function toArray()
    {
        return [
            '0' => __('Disable'), 
            '1' => __('Enable on Only Homepage'), 
            '2' => __('Enable on All Pages')
        ];
    }
}