Final answer:
The expression 13 multiplied by itself six times is written in exponential notation as 13^6, which corresponds to answer choice a.
Step-by-step explanation:
The expression in exponential notation for 13 multiplied by itself six times is 136.
This notation is concise and indicates that you should multiply the base number, which is 13 in this case, by itself as many times as the exponent indicates, which is 6 times for this question.
Therefore, the correct answer is a) 136.