Explanation:

- Let's decode the given examples:

- ‘monday february year april’ is coded as ‘bi gv oc st’.

- ‘february may of july’ is coded as ‘hm oc da pu’.

- ‘of monday september wednesday’ is coded as ‘nh mk hm gv’.

- ‘july tuesday year of’ is coded as ‘da st rx hm’.

- Identify the common words:

- February appears in the first two sentences, so 'oc' is likely for february.

- 'of' appears in the second, third, and fourth sentences, associated with ‘hm’.

- 'monday' appears in the first and third sentences, linked to ‘gv’.

- 'july' appears in the second and fourth sentences, leading us to link it with ‘da’.

- Analyze the given options:

- Option 1: gv

- Option 2: oc

- Option 3: mk

- Option 4: da

- Option 5: None of these

- The correct answer: Option 4: da