Explanation:

- The marks are in an Arithmetic Progression (AP) with the first term (a) being 4 and a common difference (d) of 3.

- The sequence for the 51 students begins as 4, 7, 10, and so on. The nth term of an AP is given by: $a_n=a+(n-1)d$.

- For 51 terms: $a_{51}=4+(51-1)\times 3=4+150=154$.

- The median in a sequence of 51 terms is the 26th term (as the number of students, 51, is odd).

- To find the 26th term: $a_{26}=4+(26-1)\times 3=4+75=79$.

From the options provided:

- Option 1: 79.5

- Option 2: 79

- Option 3: 78.5

- Option 4: 77