Select the related word from the given alternatives so that the analogy Pencil : Lead :: Pen : ? is correctly completed.

Introduction / Context:
This analogy problem focuses on the relationship between an object and the material or substance that actually performs the main function of that object. For writing instruments, it is important to distinguish between the outer body and the internal medium that produces the marks. Such word analogies test how precisely a learner understands functional relationships in everyday items.

Given Data / Assumptions:
We are given the pair Pencil : Lead. We must complete the pair Pen : ? using the same type of relation. The words refer to common writing tools and their usual parts. Exactly one option should play the same role in relation to Pen as Lead does for Pencil.

Concept / Approach:
In a pencil, the actual writing is done by the inner core called lead, while the wooden body only holds it. So the relation is instrument to its writing medium. Pen has a similar structure, where the casing holds a fluid that produces the writing on paper. To solve the analogy, we must choose the substance that performs this role in a pen.

Step-by-Step Solution:
Step 1: Understand Pencil : Lead. Lead is the dark material inside a pencil that makes marks on the paper. Step 2: Identify the relationship. The relation can be stated as writing instrument to writing medium. Step 3: Apply this idea to Pen. We need to find what plays the role of writing medium inside a pen. Step 4: Examine the options. Ball: May refer to ballpoint mechanism, but it is a small part of the tip, not the medium itself. Write: This is an action, not a substance. Blue: This is a colour and does not directly represent the writing substance. Ink: This is the fluid inside the pen that creates the marks on paper. Cap: This covers the pen but does not write. Step 5: Select the best match. Ink clearly corresponds to lead as the writing medium.

Verification / Alternative check:
We can express both pairs as parallel statements: A pencil writes using lead. A pen writes using ink. These sentences are structurally and conceptually parallel. None of the other options, when inserted into a similar sentence, yield such a natural and accurate description of how a pen functions. Therefore the analogy is preserved only when Ink is chosen.

Why Other Options Are Wrong:
Ball is part of the pen tip mechanism in ballpoint pens but is not the substance that forms the letters. Write is a verb and does not represent a component or material. Blue is merely a colour description, not the writing medium itself. Cap protects the pen tip but has no role in creating marks on paper.

Common Pitfalls:
Learners sometimes confuse any associated part of an object with the exact functional equivalent required by the analogy. In exam conditions, it is important to ask what exactly plays the same functional role for the second item that the given word plays for the first. Focusing carefully on function rather than loose association helps avoid misinterpretation and improves accuracy in such analogy questions.

Final Answer:
The analogy Pencil : Lead :: Pen : ? is correctly completed by Ink.