Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
xy_xx_yzzxx_yyy_zzxxxx

Given: xy_xx_yzzxx_yyy_zzxxxx

To fill the series, let's divide the series, xy_ / xx_yzz / xx_yyy_zz / xxxx

Let's check the options:

First option: zyzx; xyz / xxyyzz / xxzyyyxzz / xxxx (No repititive pattern is followed.)

Second option: zyxy; xyz / xxyyzz / xxxyyyyzz / xxxx (No repititive pattern is followed.)

Third option: xyxz; xyx / xxyyzz / xxxyyyzzz / xxxx (No repititive pattern is followed.)

Fourth option: zyxz; xyz / xxyyzz / xxxyyyzzz / xxxx (In each part, the number of each letter is increased by 1.)

So, the correct answer is (d).