In this missing-letters puzzle, the word is written as O_er_t_o_. Complete the English word by filling in the missing letters so that it becomes a meaningful everyday term.

Introduction / Context:
This puzzle tests your ability to recognise common English words from a pattern in which some letters are missing. Such missing-letter puzzles are very popular in competitive exams and aptitude tests because they simultaneously check vocabulary, pattern recognition and careful observation of letter positions.

Given Data / Assumptions:

• The incomplete word is written as O_er_t_o_.
• We must fill in the blanks with appropriate letters to form a meaningful English word.
• The answer must match one of the given options exactly.
• Only one option should correctly fit the pattern and form a common, valid word.

Concept / Approach:
The key idea is to treat the pattern O_er_t_o_ as a fixed sequence where some characters are already known and in fixed positions. We compare each option to this pattern, checking whether the letters in the option line up with the known letters in the pattern. The correct word will have the same length as the pattern and will place the letters O, e, r, t and o in exactly those positions where they already appear in O_er_t_o_. Vocabulary knowledge also helps us immediately recognise familiar words like "operation".

Step-by-Step Solution:
Step 1: Count the letters in the pattern O_er_t_o_. There are nine positions, some filled and some blank. Step 2: Note the fixed letters and their positions: position 1 is O, position 3 is e, position 4 is r, position 6 is t and position 8 is o. Step 3: Take the option "Operation". Write it as O p e r a t i o n and index the letters from 1 to 9. Step 4: Compare the pattern with "Operation": position 1 is O (matches), position 3 is e (matches), position 4 is r (matches), position 6 is t (matches) and position 8 is o (matches). Step 5: Check the other options. "Ordination", "Ovulation" and "Observation" do not place the letters e, r, t and o at exactly the same positions as in the pattern, so they cannot be correct. Step 6: Conclude that "Operation" is the only option that fits every constraint of the pattern and is also a very common English word.

Verification / Alternative check:
An alternative way to verify is to rewrite the pattern with underscores and try to mentally overlay each option. For "Operation", we can fill the pattern as O p e r a t i o n, and every fixed letter remains in its place. Trying to overlay "Ordination" or "Ovulation" quickly shows mismatches, for example the position where the pattern demands an e is not satisfied. This cross check confirms that "Operation" is consistent with all fixed positions and that no other option matches the structure.

Why Other Options Are Wrong:
"Ordination" does not place e, r, t and o in the same positions as required by the pattern. "Ovulation" again fails to match several fixed positions and has a different internal letter order. "Observation" is longer and misaligns with the nine character pattern, so it clearly cannot be written in the form O_er_t_o_. Hence they are all invalid answers for this specific missing-letter sequence.

Common Pitfalls:
A common mistake is to choose a word merely because it visually looks similar or because it is familiar, without carefully checking each letter position against the pattern. Another pitfall is to ignore the total length of the pattern, which immediately rules out words that are too short or too long. Some learners also try to guess by meaning alone rather than structurally matching the letters, which can lead to errors. Always remember that in missing-letter puzzles, exact position matching is the most reliable strategy.

Final Answer:
The completed word that matches the pattern O_er_t_o_ is Operation.