Answer: (iii) Common subexpression elimination

Explanation

Common subexpression elimination (CSE) detects an expression that was already computed and whose operands have not changed since, and reuses the earlier result instead of recomputing it.

Before:

x = a * b + c
y = a * b + d

After CSE:

t = a * b
x = t + c
y = t + d

The other options: loop interchange changes loop order (cache behaviour); dead-code elimination removes useless code; register allocation maps variables to registers.

$$\boxed{\text{(iii) Common subexpression elimination}}$$