In compiler terminology reduction in strength means
Answer is D
In software engineering, strength reduction is a compiler optimization where expensive operations are replaced with equivalent but less expensive operations. The classic example of strength reduction converts "strong" multiplications inside a loop into "weaker" additions – something that frequently occurs in array addressing.
Replacing run time computation by compile time computation
In compiler design strength reduction is a Machine Independent loop optimization technique.
There are expressions that consume more CPU cycles, time, and memory. These expressions should be replaced with cheaper expressions without compromising the output of expression. For example, multiplication (x * 2) is expensive in terms of CPU cycles than (x << 1) and yields the same result.
so answer is D.