You use a following the closing brace of an array initialization list.
a.
,
b.
.
c.
:
d.
;

a.

,

Explanation:

The array is used to store the multiple data with  same data type.

Syntax for initialization of array:

type name[] = {data_1, data_2, data_3,};

the comma ',' is used to separate the data with other data.

for example:

int array[] = {1,2,3,4,5};

we cannot used dot '.', colon ':' and semicolon ';' as separator.

answer; i do believe its (d) modem;

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2. wikipedia