This simple addition series adds 4 to each number to arrive at the next.

Correct Option: E

This simple addition series adds 4 to each number to arrive at the next.

This is a simple subtraction series,

Correct Option: E

This is a simple subtraction series, in which 3 is subtracted from each number to arrive at the next.

This is an alternating addition series, with a random number, 21, interpolated as every third number.

Correct Option: B

This is an alternating addition series, with a random number, 21, interpolated as every third number. The addition series alternates between adding 3 and adding 4. The number 21 appears after each number arrived at by adding 3.

In this simple addition series,

Correct Option: D

In this simple addition series, each number is 5 more than the previous number.

This is an alternating addition and subtraction series. In the first pattern, 2 is added to each number to arrive at the next;

Correct Option: D

This is an alternating addition and subtraction series. In the first pattern, 2 is added to each number to arrive at the next; in the alternate pattern, 6 is subtracted from each number to arrive at the next.