What is the main use of identities?

Correct answer: expanding and simplifying expressions quickly

The direct answer is option C: identities are mainly used to expand and simplify algebraic expressions quickly. An identity is an equality that remains true for all permitted values of its variables, such as \((a+b)^2=a^2+2ab+b^2\). Instead of multiplying every term separately, we recognise the pattern and apply the known identity. Option A, only drawing figures, belongs mainly to geometry and is not the central purpose of algebraic identities. Option B, only reading fractions, has no connection with these formulas. Option C correctly describes both important jobs: expansion changes a compact product or square into terms, while simplification collects terms into an easier form. Option D, only measuring angles, is a geometry activity and cannot describe algebraic identities. These identities save time and reduce calculation errors, but the pattern must be matched correctly. Memory cue: see a familiar algebraic shape, name its identity, then substitute carefully.