Seven persons — A, B, C, D, E, F and G — are attending an event one after another, but not necessarily in the same order. G attends the event three persons after C. B attends the event immediately before C. Only one person attends the event between B and E. D attends the event four persons after E but before A. Three persons attend event between A and F.

If all the persons are arranged in alphabetical order in the sequence from first to the last person to attend the event, then position of how many persons remains unchanged?

The correct option is One.

Let us determine the order of the seven persons (A, B, C, D, E, F, G) attending the event from 1st to 7th position based on the given clues:
1. "G attends the event three persons after C": This means there are two persons between C and G, so the arrangement relative to each other is:
Position layout: C, _, _, G.

2. "B attends the event immediately before C": This places B directly before C, giving the block:
Position layout: B, C, _, _, G.

3. "Only one person attends the event between B and E":
Since C is immediately after B, E cannot be after B. Therefore, E must be two places before B.
Position layout: E, _, B, C, _, _, G.
Since there are 7 persons in total, this completely fixes all 7 positions from 1st to 7th:
1st: E
2nd: _
3rd: B
4th: C
5th: _
6th: _
7th: G

4. "D attends the event four persons after E but before A":
Four persons after E (1st position) means D is at position 1 + 4 = 5th position.
So, 5th position = D.
Since D is before A, A must be placed after D. The remaining available positions are 2nd and 6th. Thus, A must be at the 6th position.
So, 6th position = A.

5. "Three persons attend event between A and F":
Since A is at the 6th position, F must be at position 6 - 4 = 2nd position.
So, 2nd position = F.

The final sequence of persons attending the event from 1st to 7th position is:
1st: E
2nd: F
3rd: B
4th: C
5th: D
6th: A
7th: G

Now, let us compare the original sequence with the persons arranged in alphabetical order (A, B, C, D, E, F, G):

Original Order: E, F, B, C, D, A, G
Alphabetical Order: A, B, C, D, E, F, G

Comparing position by position:
- Position 1: E vs A (Changed)
- Position 2: F vs B (Changed)
- Position 3: B vs C (Changed)
- Position 4: C vs D (Changed)
- Position 5: D vs E (Changed)
- Position 6: A vs F (Changed)
- Position 7: G vs G (Unchanged)

Only the position of G remains unchanged.

Hence, the position of only One person remains unchanged.