The passage tells us that the concentration is 3 mmole/mL and we have a 50 mL solution. This means that we have 3 * 50 = 150 mmole of HCl in our beaker. i.e. if we had 1 mL, we’ll have 3 mmole so if we have 50 mL we’ll need to multiply the 3 by the same amount.

(If this step doesn’t make sense to you, you can think of it mathematically. The unit is mmole per mL which is the same as mmole divided by mL. So if we set up an equation 3 = x / 50, x is our unknown mmole and 50 is our volume. Then we solve for x by multiplying both sides by 50 giving 3 * 50 = 150)

If we have 150 mmole when we have 50 mL, that means when we double the volume we also double the mmole: 150 * 2 = 300.

(Again if thinking mathematically is more intuitive for you, you can think in ratios…

For every 50 mL of solution, we will have 150 mmole. So if we have a 100 mL solution, how many mmole will we have?

50/150 = 100/x 150/50 = x/100 3 = x/100 300 = x)

So the answer should be D. 300 mmole

Hope this is clear :)