(a) For 7_3 < 768_: The first number is a 3-digit number, and the second number is a 4-digit number. Any 3-digit number is always less than any 4-digit number. Therefore, any digit 0-9 works in both blanks. Multiple solutions possible. (b) For 853_ < 8_3_: Both start with 8. For the second number to be larger, the hundreds digit should be $\geq$ 5. If second number has 6 in hundreds, it will be larger than first number with 5 in hundreds. (c) For 2_1 < 5_2: The first number should have thousands digit < 5, or if 5, then other digits should make it smaller. Many solutions possible.