Given that;
1 collection = 36 cards
36 cards = 4 sets at 9 card per set
Each set contains cards that are numbered 1-9

Question => what is the number on the card that was removed?

Statement 1 => The units digit of the sum of the numbers on the remaining 35 cards is 6
Sum of digits on all 9 cards = 1 +2 +3 +4 +5 +6 +7 +8 +9 = 45
Sum for all the 4 set = 4 * 45 = 180
When one card is removed, sum of the remaining 35 cards has the last digit to be = 6
180 - 1 = 179, 180 - 2 = 178, 180 - 3 = 177, 180 - 4 = 176, 180 - 5 = 175, 180 - 6 = 174
180 - 7 = 173, 180 - 8 = 172, 180 - 9 = 171
180 - 4 = 176 is the only card that will have the sum of the remaining 35 cards to be = 176 (with the last digit as 6)
Hence, number on the card removed = 4 ; statement 1 is SUFFICIENT

Statement 2 => The sum of the numbers on the remaining 35 cards is 176
Sum of digits on all 9 cards = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 +9 = 45
Sum of all the 4 sets = 4 * 45 = 180
From this statement 2 => 180 - (value of chosen card) = 176
180 - 176 = value of chosen card
chosen card = 4
Statement 2 is SUFFICIENT

Since each statement alone is SUFFICIENT
Answer = option D