Correct Answer: 42

Solution : $ND = N{_1}D{_1} + N{_2}D{_2} +N{_3}D{_3} +......+N{_n}D{_n} $, where $N$ = number of students and $D$ = number of days.
Let the number of students be $x$ initially.
$10x = x\times 1 + (x-2)\times1+(x-4)\times 1 + ......(x-28)\times1$
⇒ $10x=15x -[2+4+6+....+28]$
⇒ $5x = \frac{14}{2}[2 + 28]$
⇒ $5x=7\times30$
$\therefore x=42$
Hence, the correct answer is 42.