Fill in the blank with a suitable option:
I always enjoy __________ friends.

The correct answer is D: Visiting

Here's why:

The verb "enjoy" is often followed by a gerund. A gerund is a verb ending in "-ing" that functions as a noun.

Therefore, we need to use the "-ing" form of the verb "visit".

Let's look at the other options:

A: To visit - This is an infinitive form of the verb. While infinitives can sometimes follow verbs, "enjoy" usually takes a gerund.
B: For visiting - The word "for" is unnecessary and makes the sentence sound awkward.
C: In visiting - The word "in" is also unnecessary and not grammatically correct in this context.

So, the sentence "I always enjoy visiting friends" is the most grammatically correct and natural-sounding option.