Step 1: Understanding the types of morphological changes.
- Partial suppletion (i) occurs when a form changes completely, but only partially substitutes with an alternative form.
- Total suppletion (ii) occurs when a word has entirely different forms in the paradigm.
- Lexically conditioned (iii) refers to a change that depends on the meaning of the word.
- Phonologically conditioned (iv) refers to changes that depend on the phonetic structure of the word.
Step 2: Analyzing the options.
- P. electric $\rightarrow$ electricity: This is a case of phonologically conditioned change (iv), as the change from "electric" to "electricity" involves adding a suffix that affects the word's phonological structure.
- Q. go $\rightarrow$ went: This is an example of total suppletion (ii), where "go" and "went" are completely different forms.
- R. ox $\rightarrow$ oxen: This is an example of lexically conditioned change (iii), as the plural form "oxen" is irregular and conditioned by the meaning of the word.
- S. think $\rightarrow$ thought: This is an example of partial suppletion (i), where "think" changes to "thought" in a partially irregular way.
Step 3: Conclusion.
Thus, the correct sequence is (D) P – (iv), Q – (ii), R – (iii), S – (i).