select
Explanation:
The missing word is select, which is the keyword used to query data from a table in SQL. The select statement has the following syntax:
select column_list from table_name where condition;
The column_list can be one or more columns separated by commas, or an asterisk () to indicate all columns.
The table_name is the name of the table that contains the data. The where clause is optional and specifies a condition to filter the rows. The count() function is an aggregate function that returns the number of rows in the table or in a group. Therefore, the complete statement is:
select count(*) from tablename;
This statement will return the number of rows in the table named tablename. References: SQL COUNT() Function - W3Schools, SQL COUNT: The Ultimate Guide To SQL COUNT Function - SQL Tutorial, The SQL Count Function Explained With 7 Examples.

The valid lines in the file /etc/hosts are A and C. The format of the /etc/hosts file is as follows12:
IP_address canonical_hostname [aliases...]
where IP_address is the IPv4 or IPv6 address of the host, canonical_hostname is the official name of the host, and aliases are optional alternative names for the host. Each field is separated by whitespace (spaces or tabs).
The # character indicates the beginning of a comment, and the rest of the line is ignored.
The lines B, D, and E are invalid because they do not follow the format of the /etc/hosts file. Line B has the hostname and aliases before the IP address, which is incorrect. Line D has multiple IP addresses and hostnames separated by commas, which is also incorrect. Line E has two IP addresses for the same host, which is not supported by the /etc/hosts file. If a host has more than one IP address, it should have a separate line for each address3.
References:
* 1: hosts(5) - Linux manual page - man7.org
* 2: Format of /etc/hosts on Linux (different from Windows?)
* 3: hosts File Format for TCP/IP - IBM

groupadd, /usr/sbin/groupadd
Explanation:
The groupadd command is used to add an empty group to the system. It takes the name of the group as an argument and creates an entry for it in the /etc/group file. The groupadd command also assigns a unique group ID (GID) to the new group. The groupadd command can take various options to specify the GID, the password, and other attributes of the new group. For example, groupadd -g 1000 mygroup will create a new group named mygroup with a GID of 1000. References:
* Linux Groups - javatpoint
* groupadd(8) - Linux manual page
* How to Add and Delete User Groups on Linux