Laboratory studies using the recall of letters and numbers indicate that short-term memory is probably coded .

Verbally

Explanation:

 According to the given question, in the laboratory studies the various types of numbers and letters are recalled for the purpose of indicating that the short term memory is verbally coded.

 The short term memory is one of the type of primary memory in which the memory has the capacity for holding the data or information for the short duration.

 The main principle of the coding system is state that the short term memory basically trying the hold or recall the different types of letters and also numbers which is probably verbally coded.    

 Therefore, Verbally is the correct answer.  

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