Explanation:

Let’s break it down step by step, looking at how the letter-clusters change:

- BRAIN

(Original word. Nothing’s changed yet.)

- DRAIN

(The first letter B ? D. The rest stays the same.)

- DTAIN

(Second letter R is dropped. So now it’s D_TAIN.)

- ?

(This is the gap we’re solving for. What changes next?)

- DTCKN

(Here’s where things get interesting. The T is still there, but now the A and I are replaced by C and K.)

- DTCKP

(Final step: The last letter N ? P.)

Now, the step-by-step pattern looks like this:

1. First letter changes (B ? D)

2. Second letter removed (R missing)

3. After D T, the 'A' is about to change to C, and the I to K in next step.

4. Then, the N (last letter) changes to P.

Let’s see what’s missing between DTAIN and DTCKN:

- DTAIN ? DTCKN:

- A ? C

- I ? K

So, the missing form should keep the same pattern but just before A and I become C and K.

DT _ _ N. The only option that fits is "DTCIN" (Option 1), because:

- After DTAIN, changing A to C gets you DTCIN; changing I to K gets you DTCKN.

Now the options:

- Option 1: DTCIN  This fits. The pattern is: DTAIN ? DTCIN (A changes to C), then next step DTCIN ? DTCKN (I changes to K).

- Option 2: DTCIP (Too early for P, last letter is still N at this stage.)

- Option 3: DRCIN (Wrong, R was dropped after DRAIN.)

- Option 4: DTACN (A is yet to be replaced by C, and I by K, so this doesn’t fit.)

Option 1: DTCIN is clearly correct.