Restrict Instant Articles RSS Feed by category

wpna-restrict-rss-feed-auhor.php

<?php
if ( ! function_exists( 'wpna_restrict_feed_author' ) ) :

	/**
	 * Modifies the main RSS query.
	 *
	 * @param  WP_Query $query
	 * @return $query
	 */
	function wpna_restrict_feed_author( $query ) {

		// Requested option name, / value to default to.
		if ( function_exists( 'wpna_get_option' ) ) {
			// get the feed slug.
			$feed_slug = wpna_get_option( 'fbia_feed_slug' );

			// If it's the Instant Articles feed
			if ( $query->is_feed( $feed_slug ) && $query->is_main_query() ) {
				// Restrict to only show posts with this author ID.
				$query->set( 'author', '3' );
			}
		}

		return $query;

	}
endif;
add_filter( 'pre_get_posts', 'wpna_restrict_feed_author', 11, 1 );

wpna-restrict-rss-feed-category.php

<?php
if ( ! function_exists( 'wpna_restrict_feed_category' ) ) :

	/**
	 * Modifies the main RSS query.
	 *
	 * @param  WP_Query $query
	 * @return $query
	 */
	function wpna_restrict_feed_category( $query ) {

		// Requested option name, / value to default to.
		if ( function_exists( 'wpna_get_option' ) ) {
			// get the feed slug.
			$feed_slug = wpna_get_option( 'fbia_feed_slug' );

			// If it's the Instant Articles feed
			if ( $query->is_feed( $feed_slug ) && $query->is_main_query() ) {
				// Restrict to only show posts in a certain category.
				$query->set( 'cat', 5 );
			}
		}

		return $query;

	}
endif;
add_filter( 'pre_get_posts', 'wpna_restrict_feed_category', 11, 1 );

wpna-restrict-rss-feed-tag.php

<?php
if ( ! function_exists( 'wpna_restrict_feed_tag' ) ) :

	/**
	 * Modifies the main RSS query.
	 *
	 * @param  WP_Query $query
	 * @return $query
	 */
	function wpna_restrict_feed_tag( $query ) {

		// Requested option name, / value to default to.
		if ( function_exists( 'wpna_get_option' ) ) {
			// get the feed slug.
			$feed_slug = wpna_get_option( 'fbia_feed_slug' );

			// If it's the Instant Articles feed
			if ( $query->is_feed( $feed_slug ) && $query->is_main_query() ) {
				// Restrict to only show posts with this tag.
				$query->set( 'tag', 'action' );
			}
		}

		return $query;

	}
endif;
add_filter( 'pre_get_posts', 'wpna_restrict_feed_tag', 11, 1 );