Fill in the blank with the most appropriate option from the given one…

The correct answer is Option (3) → have been swimming

Present perfect continuous tense:

• It is used for an action that began in the past and is still going on.
• Form: has/have + been + verb-ing, usually with since (a starting point) or for (a length of time).
• Clue word: "ever since". "Ever since I was a child" gives the starting point, and the swimming goes on to the present.
• Complete sentence: I have been swimming ever since I was a child.
  Example: She has been living in Pune since 2015.

Why the Other Options are Incorrect?

Option 1: swam — Incorrect. The simple past describes a finished action; it cannot link a past starting point with the present, as "ever since" does.
Example: I swam in the river yesterday.

Option 2: am swimming — Incorrect. The present continuous describes an action going on at this moment; it is not used with a "since" period stretching from the past.
Example: I am swimming in the pool right now.

Option 4: was swimming — Incorrect. The past continuous describes an action in progress at a past moment; it does not reach up to the present.
Example: I was swimming when it began to rain.

Contrast: "ever since" joins a past starting point to the present, so it takes have been swimming; the simple past "swam" describes a finished action.