Correct Answer: + and –

Solution : Given:
551 ÷ 19 – 16 × 12 + 121 = 100

Let's check the options –
First option: – and ×
On interchanging the mathematical signs, we get –
= 551 ÷ 19 × 16 – 12 + 121
= 29 × 16 – 12 + 121
= 464 – 12 + 121
= 585 – 12
= 573 ≠ 100
Second option: + and ×
On interchanging the mathematical signs, we get –
= 551 ÷ 19 – 16 + 12 × 121
= 29 – 16 + 12 × 121
= 29 – 16 + 1452
= 1481 – 16
= 1465 ≠ 100
Third option: ÷ and ×
On interchanging the mathematical signs, we get –
= 551 × 19 – 16 ÷ 12 + 121
= 551 × 19 – 1.33 + 121
= 10469 – 1.33 + 121
= 10590 – 1.33
= 10588.67 ≠ 100
Fourth option: + and –
On interchanging the mathematical signs, we get –
= 551 ÷ 19 + 16 × 12 – 121
= 29 + 16 × 12 – 121
= 29 + 192 – 121
= 221 – 121
= 100

Only the fourth option satisfies the given equation. Hence, the fourth option is correct.