Why is わからない the wrong choice in this sentence


I came across the following exercise for JLPT N2:

親に本当のことを言おうか言うまいか＿

a わからない b ３日も考えている c 関係ない

The right answer is b, which makes sense. Though I wonder why a wouldn't also
be a reasonable choice here. Does わからない sound weird in this context, or is it
related to the tense of the verb.




It will be easier to find out if you divide the part 言おうか言うまいか.

You can divide it as 言おうか and 言うまいか.

You can omit 言うまいか because it's added just to make it clear that 言おうか is an
embedded question.

Then put each word (a, b and c) after 言おうか to see if it makes sense.

Now you notice that only the option b makes sense.

a. 親に本当のことを言おうかわからない。 → NG

b. 親に本当のことを言おうか３日も考えている。 → OK

c. 親に本当のことを言おうか関係ない。 → NG

Why "a" doesn't make sense? Because if you want to put an embedded question
before わからない, か should come after verbs in the informal form/i-adjectives
(this case for example 言った方がいいか（どうか）分からない)

言おう is the 意向形（いこうけい）(volitional form) of 言う. 意向形 of a verb + か is used to
express that the speaker is wondering whether or not he/she should do
something, and it doesn't fit well with 分からない, but it does with 考えている or
迷っている.




It will be easier to find out if you divide the part 言おうか言うまいか.

You can divide it as 言おうか and 言うまいか.

You can omit 言うまいか because it's added just to make it clear that 言おうか is an
embedded question.

Then put each word (a, b and c) after 言おうか to see if it makes sense.

Now you notice that only the option b makes sense.

a. 親に本当のことを言おうかわからない。 → NG

b. 親に本当のことを言おうか３日も考えている。 → OK

c. 親に本当のことを言おうか関係ない。 → NG

Why "a" doesn't make sense? Because if you want to put an embedded question
before わからない, か should come after verbs in the informal form/i-adjectives
(this case for example 言った方がいいか（どうか）分からない)

言おう is the 意向形（いこうけい）(volitional form) of 言う. 意向形 of a verb + か is used to
express that the speaker is wondering whether or not he/she should do
something, and it doesn't fit well with 分からない, but it does with 考えている or
迷っている.

