For answering there is no need to execute the query, we can directly answer this as $2$
How?
Group by Student_Names
It means all name that are same should be kept in one row.
There are $3$ names. But in that there is a duplicate with Raj being repeated $\implies$ Raj produces one row and Rohit produces one row $\implies$ Total $2$ rows.
For better understanding, I'll just analyze the whole query
1$^{\text{st}}$ statement which is executed from the query is From Clause $:$ From Student S, Performance P
$\implies$ cross product of those two tables will be
$$\begin{array}{|l|l|l|l|l|} \hline \textbf{S.RollNo} & \textbf{S.Student_name} & \textbf{P.Roll_no} & \textbf{P.Course} & \textbf{P.marks} \\\hline 1 & \text{Raj} & 1 & \text{Maths} & 80 \\\hline 1 & \text{Raj} & 1 & \text{English} & 70 \\\hline 1 & \text{Raj} & 2 & \text{Maths} & 75 \\\hline 1 & \text{Raj} & 3 & \text{English} & 80 \\\hline 1 & \text{Raj} & 2 & \text{Physics} & 65 \\\hline 1 & \text{Raj} & 3 & \text{Maths} & 80 \\\hline 2 & \text{Rohit} & 1 & \text{Maths} & 80 \\\hline 2 & \text{Rohit} & 1 & \text{English} & 70 \\\hline 2 & \text{Rohit} & 2 & \text{Maths} & 75 \\\hline 2 & \text{Rohit} & 3 & \text{English} & 80 \\\hline 2 & \text{Rohit} & 2 & \text{Physics} & 65 \\\hline 2 & \text{Rohit} & 3 & \text{Maths} & 80 \\\hline 3 & \text{Raj} & 1 & \text{Maths} & 80 \\\hline 3 & \text{Raj} & 1 & \text{English} & 70 \\\hline 3 & \text{Raj} & 2 & \text{Maths} & 75 \\\hline 3 & \text{Raj} & 1 & \text{English} & 80 \\\hline 3 & \text{Raj} & 2 & \text{Physics} & 65 \\\hline 3 & \text{Raj} & 3 & \text{Maths} & 80 \\\hline \end{array}$$
2$^{\text{nd}}$ statement which is executed from the query is Where Clause $:$ Where S.Roll_no = P.Roll_no
$\implies$ delete those rows which does not satisfy the WHERE condition. Then the result will be
$$\begin{array}{|l|l|l|l|l|} \hline \textbf{S.RollNo} & \textbf{S.Student_name} & \textbf{P.Roll_no} & \textbf{P.Course} & \textbf{P.marks} \\\hline 1 & \text{Raj} & 1 & \text{Maths} & 80 \\\hline 1 & \text{Raj} & 1 & \text{English} & 70 \\\hline 2 & \text{Rohit} & 2 & \text{Maths} & 75 \\\hline 2 & \text{Rohit} & 2 & \text{Physics} & 65 \\\hline 3 & \text{Raj} & 3 & \text{English} & 80 \\\hline 3 & \text{Raj} & 3 & \text{Maths} & 80 \\\hline \end{array}$$
3$^{\text{rd}}$ statement which is executed from the query is Group by Clause $:$ Group by S.Student_Name
$\implies$ Merge those rows which are having same name, then result will be
$$\begin{array}{|l|l|l|l|l|} \hline \textbf{S.RollNo} & \textbf{S.Student_name} & \textbf{P.Roll_no} & \textbf{P.Course} & \textbf{P.marks} \\\hline \{1,1,3,3\} & \text{Raj} & \{1,1,3,3\} & \text{\{Maths, English\}} & \{80,70,80,80\} \\\hline 2 & \text{Rohit} & 2 & \text{\{Maths, Physics\}} &\{75,65\} \\\hline \end{array}$$
Note that, this can't be used as final result as it violates 1NF (multiple values in each tuple for $\text{S.Roll_no}, \text{P.Roll_no}, \text{P.Course}$ and $\text{P.marks})$
4$^{\text{th}}$ statement which is executed from the query is Select Clause $:$ Select S.Student_Name, SUM(P.marks)
$\implies$ Delete un-necessary columns and calculate the aggregate functions, then result will be$$\begin{array}{|l|l|} \hline \textbf{S.Student_name} & \textbf{P.marks} \\\hline \text{Raj} & 310 \\\hline \text{Rohit} & 140 \\\hline \end{array}$$