Which of the following is/are undecidable?

To determine the undecidability of each option:

• Option A: Deciding whether $L(M_1) = L(M_2)$ for two Turing machines is the equivalence problem for TMs, which is undecidable (in fact, not Turing-recognizable / not semi-decidable).

• Option B: Testing if the language of a TM, $L(M)$, is regular is a non-trivial property of Turing-recognizable languages. ByRice's Theorem, this property is undecidable.

• Option C: Deciding if a TM accepts all strings, i.e., $L(M) = \Sigma^*$, is also a non-trivial semantic property of RE languages, and by Rice's Theorem, it is undecidable.

• Option D: In at most $1073$ steps, a deterministic Turing machine $M$ can inspect at most the first $1073$ characters of the input string. Therefore, its behavior within $1073$ steps depends only on input prefixes of length $\le 1073$. Since there are only finitely many such prefixes over a finite alphabet, we can simulate $M$ on all of them for $1074$ steps. If $M$ takes more than $1073$ steps on all these inputs, it will take more than $1073$ steps on every string. Thus, this property is decidable.

Therefore, options A, B, and C are undecidable.