Let x be the mark Ravi obtained in the third unit test.
Total marks obtained in the three test is $70+75+x$
Since the average marks of the three test in at least 60 $\large\frac{70 +75+x}{3} $$ \geq 60$
Multiplying both sides of inequality by 3 .
$145 + x \geq 180$
Subtracting 145 from both sides of the inequality.
$x \geq 180-145$
$x \geq 35$
Step 2:
The student must obtain a minimum mark of 35 to get an average of 60 marks.
Hence B is the correct answer.