Explain how to write an addition sentence for a picture of 4 rows with 3 items in each row

Any addition sentence would be a number plus a number (and any other numbers added in a sequence, if applicable) with and "equal sign", or the words "is equal to" (or, "are equal to", if applicable"), or "equals", followed by the correct number.

Or, although rarely used, any addition sentence could be written as, say:

"The sum of  "4 + 4" is equal to "8".

Any addition sentence can be written symbolically, such as:  4 + 2 = 6 .
It is an "addition sentence" because it can be "pronounced" as: 
 
"Four plus two equal six."

Also an addition sentence can correspond to a picture or diagram; the addition sentence does have to include any units. 

In the case of a picture with 4 rows with 3 items in each row, one may think this seems like a multiplication problem; e.g. "3 x 4 = 12."

However, since the question being asked requires an addition sentence, we have to write this as a sum.  We do this by using the conceptual principle that "3 *4" is like adding the number "3" (three) together, 4 (four) times.

So, the answer would be: " 3 + 3 + 3 + 3 = 12 ."