Select the set of letters that, when sequentially placed in the blanks of the given letter series, will complete the letter series.
t _ n s _ r t h _ s o _ t h _ s _ r

Let's divide the series into equal recurring blocks. The given series consists of 18 characters. Let's look for a repeating pattern of 6 letters:
t _ n s _ r∣t h _ s o _∣t h _ s _ r
By comparing the blocks:

Comparing block 1 and block 2: The 2nd position should be 'h', the 5th position should be 'o'. This gives us the root word tensor or a variation like thnsor. Let's re-verify the letters given:

If the repeated word is t h n s o r:

1st block: t [h] n s [o] r

2nd block: t h [n] s o [r]

3rd block: t h [n] s [o] r

Let's check the filled blank letters in sequence: h, o, n, r, n, o.
This matches Option A.