Step-by-step Solution:

Correct Answer: D (85)

To find the next number in the series, we first need to determine the pattern by looking at the differences between consecutive terms.

Step 1: Calculate the Differences

Let's find the difference between each term and the one that follows it:

• $210 - 195 = 15$
• $195 - 175 = 20$
• $175 - 150 = 25$
• $150 - 120 = 30$

Step 2: Identify the Pattern in the Differences

The differences themselves form a simple arithmetic progression: 15, 20, 25, 30.

The pattern is that the amount being subtracted increases by 5 with each step.

Step 3: Calculate the Next Term

Following this pattern, the next difference in the sequence should be $30 + 5 = 35$.

To find the next term in the original series, we subtract this next difference from the last known term (120):

$$ 120 - 35 = 85 $$

Therefore, the next number in the series is 85.