**Solution – **

The digits will be in ascending order -

Lets consider some initial numbers and see if we get some pattern –

In 100s -

123, 124, 125, 126, 127, 128, 129,

134, 135, 136, 137, 138, 139

145, 146, 147, 148, 149

156, 157, 158, 159

167, 168, 169

178, 179,

189

There hence will be a total of **28** numbers such numbers. (7+6+5+4+3+2+1)

Similarly

In 200s –

6+5+4+3+2+1 = 21 numbers

If we closely observe, the numbers are getting reduced while we add them and hence the next sum is –

In 300s

5+4+3+2+1 = 15 numbers

In 400s –

4+3+2+1 = 10 numbers

In 500s –

3+2+1 =6 numbers

In 600s –

2+1 = 3 numbers

In 700s –

1 number (789)

Hence a total of 28 + 21 + 15 + 10 + 6 + 3 + 1 = 84 (Option **C**)