Answer:
If the integer variable "&" holds a value between 0 and 6, we can assign 15 to the array element whose index is k using the following code:
```java
a[& * 2] = 15;
```
Explanation:
Since k should not be modified, we can use the value of "&" to calculate the index where we want to assign 15. Here, we multiply "&" by 2 to get an index between 0 and 12 (exclusive). Multiplying by 2 ensures that the index is even and will correspond to an element in the array. Finally, we assign 15 to the array element at this index.