$values = array(

        array( // option group
            'label' => 'Group 1',
            'value' =>
                array(
                    array( // Option 1
                        'value' => '1',
                        'label' => 'Option 1'
                    ),
                    array( // Option 2
                        'value' => '2',
                        'label' => 'Option 2'
                    )
                )
        ),

        array( // option group
            'label' => 'Group 2',
            'value' =>
                array(
                    array(
                        'value' => '3',
                        'label' => 'Option 3'
                    ),
                    array(
                        'value' => '4',
                        'label' => 'Option 4'
                    )
                )
        ),
    );

$allowedCurrencies = $this->scopeConfig->getValue('currency/options/allow');

<source_model>MagentoConfigModelConfigSourceLocaleCurrency</source_model>

use MagentoFrameworkAppConfigScopeConfigInterface;

/**
 * Class Config
 *
 * @package NameMyModuleModelConfigBackend
 */
class Config implements MagentoFrameworkDataOptionSourceInterface
{
    /**
     * @var MagentoFrameworkAppHelperContext
     */
    private $context;

    /**
     * @var ScopeConfigInterface
     */
    private $scopeConfig;

    /**
     * @param MagentoFrameworkAppHelperContext $context
     */
    public function __construct(MagentoFrameworkAppHelperContext $context)
    {
        $this->context = $context;
        $this->scopeConfig = $context->getScopeConfig();
    }

    /**
     * Get allowed currencies
     *
     * @return  array of allowed Currencies
     *
     **/
    public function getAllowedCurrencies()
    {
        $allowedCurrencies = $this->scopeConfig->getValue('currency/options/allow');
        $allowedCurrencies = explode(",", $allowedCurrencies);

        $options = [];
        foreach($allowedCurrencies as $k=>$v)
        {
            $options[] = ['value' => $v, 'label' => $v];
        }

        return $options;
    }


    /**
     * Admin Config action
     *
     * @return array
     */
    public function toOptionArray()
    {
        return $this->getAllowedCurrencies();
    }
}

<field id="currency" showInDefault="1" showInWebsite="0" showInStore="0" sortOrder="10"  translate="label comment" type="multiselect">
    <label>Currency</label>
    <source_model>NamespaceModulenameModelConfigSourceCurrency</source_model>
</field>

<?php
namespace NamespaceModulenameModelConfigSource;

use MagentoFrameworkOptionArrayInterface;

class Currency implements ArrayInterface {

    /**
     * @var CollectionFactory
     */
    public $currency;

    /**
     * @param MagentoCurrencySymbolModelSystemCurrencysymbol $currency
     */
    public function __construct(
        MagentoCurrencySymbolModelSystemCurrencysymbol $currency
    ) {
        $this->currency = $currency;
    }

    /**
     * Return currency symbol properties array based on config values
     *
     * @return array
     */
    protected function getCurrency()
    {
        return $this->currency->getCurrencySymbolsData();
    }

    /**
     * @return array
     */
    public function toOptionArray()
    {
        $currency = $this->getCurrency();
        $options = [];
        foreach ($this->getCurrency() as $code => $currency) {
            $options[$code] = [
                'label' => $currency['displayName'],
                'value' => $code
            ];
        }

        return $options;
    }
}