Answer: (iii) Essential complexity of software

In his famous 1987 essay "No Silver Bullet — Essence and Accidents of Software Engineering", Frederick Brooks argues that the greatest difficulty in software engineering is the essential complexity of the software itself — the inherently complex conceptual structures (the "essence" of the problem) that cannot be removed by any tool, methodology, or "silver bullet"—unlike accidental complexities (programming language quirks, tooling) which can be reduced.

Why the others are wrong:

• (i) Hardware constraints — hardware has improved dramatically; it is not the fundamental bottleneck.
• (ii) Lack of skilled developers — a real concern but not the essential difficulty Brooks highlights.
• (iv) Inadequate documentation — an accidental/practice problem, not the inherent nature of software.

Brooks concludes that no single breakthrough can achieve an order-of-magnitude improvement in productivity, reliability, or simplicity because the essential complexity — requirements, design, interlocking concepts — is inherent to software.