Slant Polarization 45 degree

No, linear polarization means that the E-vector oscillates along a line. Often this line is aligned with the Earth surface and then the polarization is named "vertical" or "horizontal". In fact , it can be aligned e.g. at 45-degrees referred to the horizon like in this case.

Dual polarization means that there are two vectors, each oscillating along its line. If they do it at one frequency, two such vectors generate elliptical polarized wave, if their amplitudes are equal and the phase shift is 90 degrees, circularly-polarized wave is radiated.

if the phase shift of the two vector is zero and vector E lies in vertical and horizontal plane (equally) then what will be the polarization that antenna ?