int length; denotes that variable length is int(integer) data type. char int; here int is a keyword cannot be used a variable name. int long; here long is a keyword cannot be used a variable name. float double; here double is a keyword cannot be used a variable name. So, the answer is int length;(Option A).
int long; here long is a keyword cannot be used a variable name.
float double; here double is a keyword cannot be used a variable name.
So, the answer is int length;(Option A).