Arrange the following words in the sequence in which they occur in the dictionary: i. Obstacle ii. Obscure iii. Obsession iv. Obstruct

Introduction / Context:
This question asks for the dictionary order of four words: Obstacle, Obscure, Obsession and Obstruct. Because they all begin with the common prefix “Obs-”, you must compare subsequent letters carefully to detect the correct alphabetical sequence. Such problems reinforce precision in letter-by-letter comparison.

Given Data / Assumptions:
- Words: Obstacle (i), Obscure (ii), Obsession (iii), Obstruct (iv).
- We assume standard English alphabetical rules.
- All words start with “Obs…”.

Concept / Approach:
Once you see that all words share “Obs” at the start, you move to the next letters to differentiate them. Compare the fourth letter, and if that is the same for some words, compare the fifth, and so on. The word with the earliest differing letter in alphabetical order goes first, and if a word is a strict prefix of another, the shorter word comes earlier.

Step-by-Step Solution:
Step 1: Write the words with key letters: Obstacle → O B S T… Obscure → O B S C… Obsession → O B S E… Obstruct → O B S T… Step 2: Compare the fourth letter (after O B S): C (Obscure), T (Obstacle), E (Obsession), T (Obstruct). Alphabetically, C < E < T. Thus Obscure (ii) must come first, then Obsession (iii), and the two “Obst…” words (Obstacle and Obstruct) come later. Step 3: Order the “Obst…” words: Obstacle vs Obstruct. Obstacle: O B S T A…; Obstruct: O B S T R… After O B S T, we compare A vs R. Since A comes before R, Obstacle (i) comes before Obstruct (iv). Step 4: Combine the order: Obscure (ii), Obsession (iii), Obstacle (i), Obstruct (iv) → ii, iii, i, iv.

Verification / Alternative check:
If you were to scan a dictionary, under “obs…”, you would first encounter “obscure” (obs-c…), then “obsession” (obs-e…), then words beginning with “obst…”, where “obstacle” (obsta…) appears before “obstruct” (obstru…). This matches the sequence ii, iii, i, iv and confirms the logic.

Why Other Options Are Wrong:
- iii, iv, i, ii puts Obsession before Obscure and Obstruct before Obstacle, both of which violate alphabetical comparisons.
- i, iv, iii, ii starts with Obstacle, even though its fourth letter T comes after C (in Obscure) and E (in Obsession).
- iv, iii, ii, i jumbles the order, ignoring the C < E < T relationship at the fourth letter.

Common Pitfalls:
Many learners see the “obst…” pattern and hastily group Obstacle and Obstruct together without considering the earlier words “obsc…” and “obse…”. Others confuse the order of C, E and T when scanning quickly. Writing down the first few differing letters and ordering them separately often makes such questions much easier.

Final Answer:
The dictionary order is Obscure, Obsession, Obstacle, Obstruct, corresponding to ii, iii, i, iv.