The correct answer is k=4.
This is due to the properties of exponents that state the following: when you multiply two like bases together, you add the exponents. Using this knowledge, we can come up with the equation: a^2 * a^k = a^6.
Because of the property we stated above, we need simplify this equation to be a^(2+k) = a^6.
Another property of exponents applies to this new equation: if you have the same bases equivalent to each other, the exponents must also be equivalent.
This means that we can simplify the above equation to be: 2+ k = 6, which when simplified, gives us our final answer of k=4.
Hope this helps!