1. Motion after the force ceases

   After the force stops acting, no force acts on the body, so it moves with constant velocity.

   It covers $50\,\text{m}$ in the next $10\,\text{s}$, so its velocity at the moment the force ceased was

   $v = \frac{50}{10} = 5\,\text{m/s}$

2. Motion during the first 20 s

   The body starts from rest, so

   $u = 0$

   The force acts for $20\,\text{s}$, producing acceleration $a$. Hence,

   $v = u + at$

   $5 = 0 + a(20)$

   $a = \frac{5}{20} = 0.25\,\text{m/s}^2$

3. Use Newton's second law

   $F = ma$

   Given mass $m = 20\,\text{kg}$,

   $F = 20 \times 0.25 = 5\,\text{N}$

4. Check options

   • A: $40\,\text{N}$ ❌
   • B: $20\,\text{N}$ ❌
   • C: $5\,\text{N}$ ✅
   • D: $10\,\text{N}$ ❌

Therefore, the correct answer is Option C.