In this one-word substitution question, choose the single English word that best expresses the phrase "situated at the base", referring to something located at or forming the bottom part of a structure or organ.

Introduction / Context:
This question assesses your understanding of a technical descriptive adjective often encountered in biology, anatomy, and other sciences. The phrase "situated at the base" is frequently used in textbooks to describe the location of a part or feature. English offers a precise word for this positional description, and recognising it can be very helpful when reading scientific material or answering objective questions that use such terminology.

Given Data / Assumptions:
We are dealing with the phrase "situated at the base". We must choose the single adjective that means located at or forming the base. We assume the question refers to standard scientific English usage.

Concept / Approach:
The word "basal" comes from "base" and is used to mean "relating to or forming the base; situated at the base". It is common in expressions such as "basal cell", "basal ganglia", or "basal layer". It directly answers the phrase given in the question. The other options either have different specialised meanings or are not standard English words in this sense.

Step-by-Step Solution:
Step 1: Identify that the phrase describes a position, not a person or action. Step 2: Note the key word "base" and recall related adjectives like "basal". Step 3: Check each option's meaning; only "basal" is derived from "base". Step 4: Confirm that dictionaries define "basal" as "forming or situated at the base". Step 5: Select Option D as the correct one-word substitution.

Verification / Alternative check:
To verify, imagine a biology description: "The basal part of the leaf attaches to the stem." Here, "basal" clearly indicates the part at the bottom or base. If you substituted "dorsal", "tussal", or "versal" here, the sentence would not make sense or would be incorrect. This confirms that "basal" is the only option that accurately represents the phrase in the question.

Why Other Options Are Wrong:
Option A, "Dorsal", means relating to the back or upper side of an animal or organ, as in "dorsal fin", not the base. Option B, "Tussal", is not a recognised standard English adjective with the required meaning and appears incorrect in this context. Option C, "Versal", likewise does not represent the idea of being at the base and is not the accepted term in scientific or general English. None of these words is used to mean "situated at the base".

Common Pitfalls:
Students sometimes mix up positional adjectives like "basal", "dorsal", "ventral", and "lateral" because they encounter them mainly in biology diagrams without fully learning their definitions. Another pitfall is guessing based on sound, picking a word that "feels" right instead of recalling the root. A good strategy is to connect "basal" firmly with "base" in your memory and to revise common positional terms together so that their differences become clear.

Final Answer:
The correct answer is "Basal".