Magento 2 - Drop down select list for custom magento system configuration section

Try this code.

system.xml

<field id="list_mode" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
   <label>List Mode</label>        
   <source_model>Vendor\Module\Model\Config\Source\ListMode</source_model>
</field>

Vendor\Module\Model\Config\Source\ListMode.php

namespace Vendor\Module\Model\Config\Source;

class ListMode implements \Magento\Framework\Data\OptionSourceInterface
{
 public function toOptionArray()
 {
  return [
    ['value' => 'grid', 'label' => __('Grid Only')],
    ['value' => 'list', 'label' => __('List Only')],
    ['value' => 'grid-list', 'label' => __('Grid (default) / List')],
    ['value' => 'list-grid', 'label' => __('List (default) / Grid')]
  ];
 }
}

I got solution. Below line of code will work fine.

System.xml

            <field id="select" translate="label" type="select"
                sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>Select Option</label>
            <source_model>Vendor\Modulename\Model\Config\Source\Products</source_model>
            </field>

Model file

Vendor\Modulename\Model\Config\Source;

use Magento\Framework\Option\ArrayInterface;

class Products implements ArrayInterface
{

/*
 * Option getter
 * @return array
 */
public function toOptionArray()
{
    $arr = $this->toArray();
    $ret = [];
    foreach ($arr as $key => $value) {
        $ret[] = [
            'value' => $key,
            'label' => $value
        ];
    }
    return $ret;
}

/*
 * Get options in "key-value" format
 * @return array
 */
public function toArray()
{
    $choose = [
        '1' => 'A',
        '2' => 'B',
        '3' => 'C',
        '4' => 'D'

    ];
    return $choose;
}

}

As Magento\Framework\Option\ArrayInterface has been deprecated since v.102.0.1, I'm posting it in the current standard adopted by Magento (I'm using 2.3.3 and PHP 7.3):

<?php

namespace Vendor\Package\Model\Config\Source;

use Magento\Framework\Data\OptionSourceInterface;

/**
 * Class Profile
 * @package Vendor\Package\Model\Config\Source
 */
class Profile implements OptionSourceInterface
{
    /**
     * @return array
     */
    public function toOptionArray() : array
    {
        return [
            ['value' => '', 'label' => __('-- Select an Option --')],
            ['value' => 'grid', 'label' => __('Grid Only')],
            ['value' => 'list', 'label' => __('List Only')],
            ['value' => 'grid-list', 'label' => __('Grid (default) / List')],
            ['value' => 'list-grid', 'label' => __('List (default) / Grid')]
        ];
    }
}