Hi Dennis,
Here is the example

function buddydev_validate_field_dennis() {

	$field_1 = 12;//the first field which has two options
	//since field one is required, we don't validate if it is not present, Bp will take care of it
	if ( ! isset( $_POST[ $field_1 ] ) ) {
		return;
	}

	$value = $_POST[ $field_1 ];////make sure to escape it , I don't know expected type, so have left it.

	//if we are here, one of the option has been selected, so

	$bp = buddypress();

	if ( $value == 'a' ) {
		//check if question 2( assume field id 13) is present
		if ( empty( $_POST['field_13'] ) ) {
			$bp->signup->errors['field_13'] = 'This is a required field, you should not askip it';
		}

	} elseif ( $value == 'b' ) {

		//in this case, field_14, field_15, field_16 is required
		$required_fields = array( 14, 15, 16 );
		foreach ( $required_fields as $field_id ) {
			if ( empty( $_POST[ 'field_' . $field_id ] ) ) {
				$bp->signup->errors[ 'field_' . $field_id ] = 'Required field.';
			}

		}

	} else {
		//not of the allowed value
		$bp->signup->errors[ $field_1 ] = 'Please make sure to use a valid value.';
	}
}

add_action('bp_signup_validate', 'buddydev_validate_field_dennis' );

You will need to update the field ids and the value of the first field( ‘a’, ‘b’ with actual values)

I hope you can take it from there.

Regards
Brajesh