Option C

Error part – two legs

Explanation:

feet will replace two legs because - stand on one's own feet (Idiom): to be independent and able to take care of yourself.

Look at the examples given below:

When his parents died, he had to learn to stand on his own feet.

You have to learn to stand on your own feet and not always listen to your friends.

Option B

Error part –  to travel

Explanation:

travelling will replace to travel because -avoid (Verb) always agrees with a Gerund (- ing Verb) and not an Infinitive.

Look at the examples given below:

She avoids to do (Inf.) her homework. (X)

She avoids doing (Gerund) her homework. (✓)

Option C

Error part – that are interesting

Explanation:

is will replace are because -when a Plural Noun is placed with a Singular Subject (one, each, etc.) with of, the Verb will be Singular.

Look at the example given below:

Only one of his employees is sincere at work.

Only one of his students is awarded the 'Scholar award'.

Hence, that is interesting is the right usage.

Option B

Error part – to have

Explanation:

having will replace to have because –

We use the Gerund (- ing Verb) for an action that happens before or at the same time as the action of the Main Verb.

We use the Infinitive for actions that follow the action of the Main Verb.

Look at the examples given below:

I enjoy myself (at the time of) playing.

I deny having stolen anything (before).