add_filter( 'woocommerce_product_add_to_cart_text' , 'custom_woocommerce_product_add_to_cart_text' );
function custom_woocommerce_product_add_to_cart_text() {
    global $product;    
    $product_type = $product->product_type;  
    switch ( $product_type ) {
case 'variable':
            return __( 'Options', 'woocommerce' );
        break;
}
}

function my_custom_cart_button_text( $text, $product ) {
    if( $product->is_type( 'variable' ) ){
        $text = __('Buy Now', 'woocommerce');
    }
    return $text;
}
add_filter( 'woocommerce_product_single_add_to_cart_text', 'my_custom_cart_button_text', 10, 2 );