ANSWER
The explicit rule for the sequence is 

EXPLANATION
The terms of the sequence are 

The first term of the sequence is 

and the constant difference is 

Since there is a constant difference, it means the sequence is an arithmetic progression.
The general formula for an arithmetic sequence is given by the formula,

We substitute the above values to obtain,

We expand to obtain,

This will simplify to,

Therefore the correct answer is A.