what is the partial pressure of gas D in kPa (kilopascals)?

1 atm = 760 mmHg = 101.3 kPa

Total pressure, P(Total) = (3350 mmHg) × [(101.3 kPa)/(760 mmHg)] = 446.5 kPa
Partial pressure of gas A, P(A) = 95.0 kPa
Partial pressure of gas B, P(B) = (0.250 atm) × [(101.3 kPa)/(1 atm)] = 25.3 kPa
Partial pressure of gas C, P(C) = (950 mmHg) ×[(101.3 kPa)/(760 mmHg)] = 126.6 kPa

P(Total) = P(A) + P(B) + P(C) + P(D)
Partial pressure of D, P(D) = P(Total) - P(A) - P(B) - P(C) = (446.5 - 95.0 - 25.3 - 126.6) atm = 250.2 kPa

(The answer may be slightly different is 101 kPa is used instead of 101.3 kPa.)

Convert the pressures of all gases to the same units (kPa would make the most sense).

Total pressure = sum of partial pressures of all gases.

Just subtract partial pressures of the other gases from the total to find partial pressure of D.