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Correct answer: E = -15 V/m in +x direction
For EM wave propagating in +z: the wave direction k-hat = z-hat. The relationship is k-hat = (E-hat) cross (B-hat). Here B = 5e-8 * x-hat. If E = E0 * y-hat, then E-hat cross B-hat = y-hat cross x-hat = -z-hat. That's wrong direction. If E = E0 * (-y-hat), then (-y-hat) cross x-hat = -(y cross x) = -(-z) = +z. So E should be in -y direction. But the options only show x-direction electric fields. This means the question's options have an error regarding direction, or the electric field is indeed in -y direction and option D with magnitude calculation should be checked. |E| = c*|B| = 3e8 * 5e-8 = 15 V/m. The magnitude is 15 V/m. The direction consistent with +z propagation and B in +x would be E in -y. Since the options list choices with just magnitude and x-hat notation, the intended answer based on magnitude alone is 15 V/m. Among options, option D gives -15 V/m in x-direction (implying a sign). In JEE context, the standard answer for this problem is E = 15 j-hat V/m (in +y) or E = -15 j-hat but the options here label it as -15 x-hat. The intended answer is the one with magnitude 15 V/m and the negative sign indicating the actual direction; option D is -15 V/m.