Explanation:
The correct preposition to fill in the blank is "on." Here is the explanation for why "on" is the correct preposition in this sentence:

Meaning of the Sentence:
The sentence states that the spider is crawling in relation to the wall, indicating its location or movement.

Choosing the Correct Preposition:
To choose the correct preposition, we need to consider the relationship between the spider and the wall. Let's examine the options:

a) into: This preposition implies movement from the outside to the inside of something. In the given sentence, the spider is not entering or going inside the wall; it is moving along its surface. Therefore, "into" is not the correct preposition.

b) on: This preposition indicates the surface or location where the spider is crawling. In the given sentence, the spider is crawling on the wall, which accurately describes its movement. Hence, "on" is the correct preposition for this sentence.

c) along: This preposition suggests movement in a line or direction that is parallel to something. While the spider is indeed moving in a line or direction (up, down, or sideways), the sentence does not provide enough information to indicate that the spider is moving parallel to the wall. Therefore, "along" is not the correct preposition in this context.

d) under: This preposition signifies movement below or beneath something. In the given sentence, the spider is not crawling beneath the wall; it is crawling on the wall's surface. Consequently, "under" is not the correct preposition.

Conclusion:
Based on the analysis of the given options and the context of the sentence, the correct preposition to fill in the blank is "on." This choice accurately describes the movement of the spider on the surface of the wall.