In this question, we have to find the molar concentration of the acid, and the question already provides us with the formula and each required data:
MaVa = MbVb
We have:
Ma = ?
Va = 100 mL
Mb = 1.0 M
Vb = 25 mL
Now we add these values into the formula:
Ma * 100 = 1 * 25
100Ma = 25
Ma = 25/100
Ma = 0.25 M