Explanation:

For 1 to 9 :
9 times
for 10 to 99 :
2 × (99-10+1)
=2×90 =180 times
for 100 to 300 :
3 × (300-100+1)
=3×201 =603 times
total no.of times keys should be pressed
= 9 + 180 + 603 = 792 times

Hence, option 4 is the correct answer.