Option C: $\dfrac{3}{4}$
When two coins are tossed, total outcomes = {HH, HT, TH, TT} = 4. Favourable outcomes (at least one head) = {HH, HT, TH} = 3. So, P(at least one head) = $\dfrac{3}{4}$.
Source: Chapter 14, Example 9
"At least one head" means one or more heads — include all outcomes except TT. Students often wrongly count only 3 outcomes total (HH, HT/TH, TT), giving $\dfrac{1}{3}$; remember HT and TH are different outcomes.