Introduction:
In this question, we are asked to identify the meaning of an arrow mark at the end of a line. We are given five options to choose from, and we need to select the correct one.

Explanation:
The arrow mark at the end of a line represents that the line can be extended. Let's understand why this is the correct answer.

Meaning of an arrow mark at the end of a line:
When we see an arrow mark at the end of a line, it indicates that the line can be extended further. The arrow points in the direction in which the line can be extended.

Example:
To better understand this concept, let's consider an example. Suppose we have a line segment AB with an arrow mark at point B. The arrow is pointing towards the right side. This means that the line segment AB can be extended further towards the right.

Significance of the arrow mark:
The arrow mark is used to indicate the direction in which the line can be extended. It helps us visualize the line's potential continuation beyond its current endpoint.

Options given in the question:
Let's analyze the options given in the question to determine which one is correct:

a) Can be extended: This option aligns with the meaning of an arrow mark at the end of a line. When we see an arrow, it suggests that the line can be extended further.

b) Cannot be extended: This option contradicts the meaning of an arrow mark. If there is an arrow at the end of a line, it indicates that the line can be extended.

c) Can be extended towards the opposite direction: This option is not correct because the arrow mark points in the direction of extension, not towards the opposite direction.

d) All the above: This option is not correct as options b and c are incorrect.

e) None of these: This option is not correct as option a is the correct answer.

Conclusion:
Based on the explanation above, we can conclude that an arrow mark at the end of a line represents that the line can be extended. Therefore, the correct answer is option a) Can be extended.