The sentence “He is not interested ____ cycling.” uses the preposition “in.” The correct form is “He is not interested in cycling.”

The preposition “in” is used to show involvement, connection, or attraction to an activity. When we talk about hobbies, habits, or areas of interest, we use “interested in.” For example, “She is interested in music” or “They are interested in sports.”

In English grammar, adjectives like interested, engaged, involved, and successful are often followed by certain prepositions. The adjective “interested” always takes the preposition “in” when expressing what someone likes or wants to know about.

Other options do not fit the rule:
For shows purpose, not interest. Example: “This gift is for you.”
With shows association or accompaniment. Example: “He went with his friends.”
At shows a specific point or skill. Example: “He is good at football.”

So, none of them can correctly express interest in an activity. The only suitable preposition is “in.”

When we say “interested in”, it connects the subject to a noun or gerund (the –ing form of a verb). In this case, “cycling” is a gerund, meaning “the activity of riding a bicycle.” Therefore, “He is not interested in cycling” means he does not enjoy or like riding a bicycle.

Here are some related examples to understand the structure:
I am interested in reading books.
She is interested in learning English.
They are interested in playing cricket.

Each of these uses the same grammatical pattern:
Subject + be verb + interested in + noun/gerund.

This pattern helps express likes, dislikes, or curiosity about something. In spoken and written English, it is one of the most common adjective-preposition combinations.

According to Oxford English Grammar and Cambridge Dictionary, the preposition “in” after “interested” is the only grammatically correct form. It connects the feeling of curiosity or enjoyment with the activity or topic.

Therefore, the correct and complete sentence is:
He is not interested in cycling.

This sentence means that cycling does not attract him or he has no desire to take part in it