Answer:

Explanation:
The correct answer is option 'a' - "into". The river Yamuna flows into the Ganges in Allahabad.

Definition of Collocation:
Collocation refers to a combination of words that are often used together and have become established as a conventional expression. It is the habitual juxtaposition of a particular word with another word or words with a frequency greater than chance.

Explanation of the Collocation:
In this sentence, the collocation "flows into" is used to describe the movement of the river Yamuna towards the Ganges in Allahabad. Here, "flows" refers to the movement of water in a river, and "into" denotes the direction or destination where the river is moving.

Sentence Analysis:
The sentence states that the river Yamuna flows into the Ganges in Allahabad. Let's analyze the given options to determine the most suitable collocation:

a) Into: This option matches the correct collocation "flows into" and denotes the movement of the river towards the Ganges. It accurately portrays the direction of the river's flow.

b) In: This option does not convey the correct meaning of the river's movement towards the Ganges. "In" usually refers to something being inside or contained within, which is not the intended meaning in this sentence.

c) From: This option does not convey the correct meaning of the river's movement towards the Ganges. "From" usually refers to the starting point or origin of something, which is not applicable in this context.

d) Between: This option does not convey the correct meaning of the river's movement towards the Ganges. "Between" is used to describe the relationship or position of two or more things, which is not relevant to the sentence.

Conclusion:
Considering the given options, the most suitable collocation for the sentence is "flows into". It accurately describes the river Yamuna's movement towards the Ganges in Allahabad.