The instruction that should replace /* Missing Loop Header */ so that the code segment works as intended is (c) counter < num.length()
The program is an illustration of loops (the while loop)
While loops are program statements that are used to perform repeated operations
For the string to return 23456, the while loop body must be repeated as long the counter is less than the length of the string num
Hence, the missing loop header is counter < num.length()
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