Arrange the following living organisms in a logical order of biological complexity (simplest to most complex): (a) Amoeba (b) Oyster (c) Worm (d) Cow. Choose the correct sequence that progresses from a unicellular organism to increasingly complex multicellular animals.

Introduction / Context:
This verbal reasoning item tests your ability to arrange biological entities in a logical progression of structural complexity—from the simplest form of life listed to the most complex. Recognizing the relative complexity of organisms (unicellular vs. multicellular, invertebrate vs. vertebrate) is the key skill.

Given Data / Assumptions:

• (a) Amoeba — unicellular protozoan
• (b) Oyster — mollusk (invertebrate)
• (c) Worm — simple invertebrate (e.g., annelid/nematode)
• (d) Cow — mammal (vertebrate)

Concept / Approach:
Order by increasing organizational complexity. A unicellular organism lacks tissues and organs. Multicellular invertebrates have tissues and, increasingly, organ systems. Vertebrates exhibit the highest level in this list, with complex organ systems and centralized nervous structures. Between worm and oyster, both are invertebrates, but a bilaterally symmetrical, organ-system-level worm is generally placed before a mollusk (oyster) when arranging a simple-to-complex, animalia-centric ladder in elementary reasoning sets.

Step-by-Step Solution:
Step 1: Identify the simplest form — Amoeba (unicellular) → start with (a).Step 2: Next, choose the simplest multicellular invertebrate — Worm (c).Step 3: Move to a more complex invertebrate — Oyster (b), a mollusk with specialized structures.Step 4: End with the most complex vertebrate listed — Cow (d).

Verification / Alternative check:
Check if each transition increases complexity (cellularity → tissue/organs → advanced systems). The sequence a → c → b → d satisfies this. Reversing any adjacent pair (e.g., placing oyster before worm) can be arguable in biology curricula, but typical reasoning questions adopt the above hierarchy from elementary to more complex body plans.

Why Other Options Are Wrong:
(b) puts oyster before worm; (c) reverses the scale; (d) begins at an intermediate organism; (e) contradicts the feasible graded path. None match the simple-to-complex rationale as cleanly as a, c, b, d.

Common Pitfalls:
Confusing “size” with “complexity” (oysters can be large but are invertebrates). Also, forgetting that a unicellular amoeba must come first.

Final Answer:
a, c, b, d